CATALYTIC GENERATION AND STORAGE OF HYDROGEN FROM ... · and storage of hydrogen from hydrolysis of...

CATALYTIC GENERATION AND STORAGE OF HYDROGEN

FROM HYDROLYSIS OF SODIUM-BOROHYDRIDE

UNDER PRESSURE

APPLICATION IN A HYDROGEN/OXYGEN FUEL CELL

by

Maria Josefina Figueira Ferreira

A dissertation submitted to the Department of Chemical Engineering

and to the Department of Mechanical Engineering,

Faculty of Engineering from University of Porto, Portugal,

in conformity with the requirements for the degree of

Master in Fundamentals and Applications of Fluid Mechanics

This dissertation was supervised by

Dr. Alexandra Maria Pinheiro da Silva Ferreira Rodrigues Pinto,

CEFT, Department of Chemical Engineering,

Faculty of Engineering of the University of Porto, Portugal

November 2009

iii

To my son,

Luís

To the memory of my dearly loved grandfather,

José da Silva

v

PREFACE

This dissertation on hydrogen generation from catalytic hydrolysis of sodium borohydride

has grown out from the necessity of finishing my Master course in Fundamentals and

Applications of Fluid Mechanics, started on the year 2003, at FEUP. After a gap of four

years, in early September 2007, I had the privilege of speaking with Dr. Carlos T. Pinho,

the Head of the Master course, who offered me the opportunity of working on EDEN’s

Project (http://www.h2eden.com) – Task PR07.33.01 – Hydrogen generation via chemical

hydrides, on a 8.5 months scholarship, funded by the Agência de Inovação S.A. of

Portugal. This was my first experience on the interesting field of clean energies! Dr.

Alexandra Pinto, the scientific leader of Task PR07.33.01, with whom I’ve been working

since 15 of October 2007, gently accepted my invitation to be the supervisor of this

dissertation and I am deeply thankful for that.

Hydrogen generation and storage from chemical hydrides, specifically from sodium

borohydride, to fuel hydrogen/oxygen fuel cells is a vast and rapidly evolving research

field since the late 1990s. This dissertation does not provide an exhaustive review; my

ambition was to write a text that describes one year and half of intensive experimental

work and that shows my delight in understanding the catalysed hydrolysis reaction of

sodium borohydride (NaBH4), thus closing the old objective of finishing my Master

course.

The start of this investigation turned out to be a hard work, because my previous

experience on the subject was scarce; but I have been fortunate in having a supervisor who

gave me all the support and freedom as the research evolved. Beginning with the aim of

“producing and simultaneously storing molecular hydrogen due to solubility’s effects in

the remaining solution after NaBH4 hydrolysis completion in a batch reactor” (see Pinto et

al., Int. J. Hydrogen Energy, 31 (2006) 1341-47), the work progresses following three

main lines of kinetic experiments: alkali, alkali free and less Polar Organic Polymeric

Solutions (lPOPS) hydrolysis of sodium borohydride under pressure. Therefore, three main

individual chapters where written, in which the experimental results are shown and

vi

compared with published work. In this dissertation, most chapters start with a specific

related literature review, and the need for a separate chapter of literature background in the

mainly body text was not felt. In this sense, the information in each individual chapter can

be understood without the need to read the precedents ones. It is my hope that the readers

understand the lines beneath this dissertation and recognize the importance, which is

particularly high in portable applications, of producing hydrogen via sodium borohydride

hydrolysis, with the goal of feeding a polymer electrolyte membrane (PEM) fuel cell on

demand.

At last, a final word for Hydrogen: being the simplest and the most abundant atom both on

the Universe and in Earth and having the highest energy to weight ratio (12.24×104 kJ/kg)

of all the energy sources known today, are not prime reasons to change energetic

paradigm?

M. Josefina F. Ferreira

vii

ABSTRACT

Ferreira, M.J.F., University of Porto, Faculty of Engineering, November 2009. Catalytic generation and storage of hydrogen from hydrolysis of sodium borohydride solutions under pressure – application in a hydrogen/oxygen fuel cell. Supervisor: Dr. Alexandra M.F.R. Pinto.

The present dissertation aimed at studying the catalytic generation and storage of hydrogen

from hydrolysis of sodium borohydride solutions under pressure. The catalytic hydrolysis

of sodium borohydride (NaBH4) was studied under pressure, up to 2.6 MPa, using a reused

ruthenium nickel based powered catalyst, in three batch reactors with internal volumes of

0.645, 0.369 and 0.228 l, made of stainless-steel and positioned vertically. Three different

types of sodium borohydride hydrolysis were considered, two of them in the presence of an

inhibitor (sodium hydroxide), namely, alkali hydrolysis and also less Polar Organic

Polymeric Solutions (lPOPS), and the other in absence of sodium hydroxide, designated by

alkali free hydrolysis. The effects of temperature, sodium borohydride concentration,

sodium hydroxide concentration, catalyst concentration and system pressure, on the

hydrogen gas generation rate, were investigated. Particular importance was given to the

effects of reactor bottom geometry on hydrogen generation yields, rates and induction

times. Two different geometries were adopted for this purpose - a flat and a conical bottom

shapes. The reactor with the conical shape significantly increases the reaction rates and

decreases the lag time. Successive loadings of reactant solution with and without magnetic

stirring were also presented to evaluate the capability of generating H2 without recharging

the reactor with fresh catalyst.

A detailed characterization of ruthenium nickel based catalyst after 200 reutilizations

is presented in terms of textural properties and of morphology. The results reveal some

deterioration of the catalyst which leads to slower rates and lower hydrogen yields, but

with similar lag time values.

Analysis of reaction by-product by X-Ray Diffractometry revealed the presence of

sodium metaborate in alkali hydrolysis and anhydrous borates in lPOPS hydrolysis. The

formation of anhydrous borates is particularly important because it increases the

gravimetric hydrogen storage density.

During the entire experimental studies, the hydrogen generated by the catalytic

hydrolysis of sodium borohydride was used to fuel a PEM fuel cell available at the CEFT

research Laboratory to evaluate the capabilities of an integrated solution/application.

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RESUMO

Ferreira, M.J.F., Universidade do Porto, Faculdade de Engenharia, Novembro 2009. Produção e contentorização de hidrogénio a partir da reacção de hidrólise catalizada de borohidreto de sódio sobre pressão – aplicação numa célula de combustível a hidrogénio/.oxigénio Supervisora: Professora Doutora Alexandra M.F.R. Pinto.

O objectivo principal desta dissertação foi o estudo detalhado da produção e

contentorização de hidrogénio a partir da reacção de hidrólise catalizada de borohidreto de

sódio sobre pressão A hidrólise catalisada de borohidreto de sódio foi estudada sobre

pressão, até 2.6 MPa, recorrendo a um catalisador em pó à base de níquel, impregnado com

ruténio, não suportado, em três reactores de aço inox, com funcionamento por partidas e

posicionamento vertical, com volumes internos 0.645, 0.369 and 0.228 l, respectivamente.

Três tipos de hidrólise foram considerados – duas delas ocorrem na presença de um

inibidor alcalino (hidróxido de sódio) e são designadas como hidrólise alcalina e como

hidrólise de solução orgânica menos polar (lPOPS); a terceira ocorre sem a presença deste

inibidor e é designada por hidrólise sem hidróxido. Os efeitos da temperatura,

concentração do borohidreto de sódio, concentração de hidróxido de sódio, concentração

de catalizador e pressão do sistema, na taxa de geração de hidrogénio molecular foram

investigados. Foi dada particular atenção aos efeitos da geometria do fundo do reactor no

rendimento, taxa e tempo de indução reacional. Duas geometrias diferentes para o fundo

do reactor foram selecionadas com este propósitio: uma plana e a outra cónica.

Alimentações sucessivas de solução reagente com e sem agitação magnética foram usadas

para avaliar a actividade do catalizador.

O catalizador de ruténio à base de níquel após 200 re-utilizações foi caracterizado em

termos de textura e morfologia. Os resultados revelaram alguma deterioração do

catalizador, traduzindo-se em taxas de produção de hidrogénio e rendimentos de reacção

mais baixos, mantendo-se no entanto os tempos de indução.

A análise do produto da reação por difracção de raios X revelou a presença de

metaborato de sódio na hidrólise alcalina e de boratos de sódio anidridos na hidrólise de

solução orgânica menos polar (lPOPS).

O hidrogénio gerado por hidrólise catalítica de borohidreto de sódio foi usado para

alimentar uma célula de combustível de membrana de permuta cateónica, disponível no

laboratório do CEFT, para analisar a possibilidade de soluções/aplicações integradas.

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ACKNOWLEDGEMENTS

I would like to acknowledge the friends and colleagues at Department of Chemical

Engineering at FEUP for their help during this work. In particular, I would like to mention:

Dr. Alexandra M. P. S. F. R. Pinto for her supervision.

Dr. Carlos T. Pinho for the opportunity of introducing me to the very interesting theme of

‘the clean energies, through the conception of 8.5 months scholarship (15Out07 to

30Jun08) of the EDEN’s Project – INEGI Collaboration, financed by ADI of Portugal,

under which a great part of this work was carried out.

Dr. Carmen Rangel, from INETI, for catalyst conception and supplying.

Dr. Luís Gales, from IBMC-ICBAS, for his active collaboration in performing the XRD

analysis.

Dr. Carlos P. M. de Sá, from CEMUP, for working on XPS-SEM/EDS catalyst

characterization, after 200 times of being reused.

Dr. João B. L. Moreira de Campos, for the payment of XPS-SEM/EDS analysis.

Dr. Manuel Fernando R. Pereira, from LCM-DEQ-FEUP, for working on N2 adsorption

isotherms (BET) catalyst characterization, after 200 times of being reused.

Eng. Luís Carlos Matos for helping with the data acquisition system.

Special thanks to members of CEFT-DEQ-FEUP for laboratory facilities.

I also would like to express my thankfulness to my husband, Pedro Ribeiro, for his

constructive criticisms.

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LIST OF CONTENTS

DEDICATION III

PREFACE V

ABSTRACT VII

RESUMO IX

ACKNOWLEDGEMENTS XI

LIST OF CONTENTS XIII

LIST OF FIGURES XVII

LIST OF TABLES XXI

NOMENCLATURE XXIII

1 INTRODUCTION 1 1.1 General introduction / 1 1.1.1 Hydrogen as energy carrier / 1 1.1.2 Chemical hydrides as H2 carrier: the particular case of NaBH4 / 4 1.2 Hydrogen/oxygen fuel cell / 7 1.2.1 Polymer Electrolyte Membrane Fuel Cell (PEMFC) / 8 1.2.2 The “MicroBoro Bus” – a didactic demonstration prototype / 10 1.3 Objectives and organization of the dissertation / 11 1.4 Conclusions / 13 1.5 References / 13 2 EXPERIMENTAL TECHNIQUES AND APPARATUS 15 2.1 Introduction / 15 2.2 Materials / 15 2.2.1 Sodium borohydride / 15 2.2.2 Nickel based bimetallic catalyst / 16 2.3 Apparatus for kinetic studies / 19 2.3.1 Reactor design / 19 2.3.2 Experimental rig / 21 2.4 General plan for sodium borohydride hydrolysis experiments / 22 2.4.1 Alkali hydrolysis / 22 2.4.1.1 Successive loadings of reactant solution / 23 2.4.2 Alkali free hydrolysis / 23 2.4.3 Less Polar Organic Polymeric Solutions (lPOPS) hydrolysis / 24 2.5 Reaction products analysis / 25 2.5.1 Molecular hydrogen / 25 2.5.2 Sodium borates by-products / 25 2.6 Conclusions / 26 2.7 References / 26

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3 ALKALI HYDROLYSIS Results for sodium borohydride kinetic experiments

27

3.1 Introduction / 27 3.2 Experimental procedure / 32 3.2.1 Effect of reactor temperature on hydrogen generation rate / 32 3.2.2 Effect of NaBH4 concentration on hydrogen generation rate / 33 3.2.3 Effect of NaOH concentration on hydrogen generation rate / 33 3.2.4 Effect of pressure on hydrogen generation rate / 33 3.2.5 Effect of reactor bottom shape on hydrogen generation rate / 34 3.2.6 Effect of successive loadings on catalyst activity/ 34 3.2.6.1 Effect of agitation on hydrogen reaction rate / 35 3.3 Results and discussion / 35 3.3.1 Effect of reactor temperature on hydrogen generation rate / 35 3.3.2 Effect of NaBH4 concentration on hydrogen generation rate / 36 3.3.3 Effect of NaOH concentration on hydrogen generation rate / 37 3.3.4 Effect of pressure on hydrogen generation rate / 37 3.3.5 Effect of reactor bottom shape on hydrogen generation rate / 38 3.3.6 Effect of successive loadings on catalyst activity / 39 3.3.6.1 Effect of agitation on hydrogen reaction rate / 42 3.4 Reaction by-product characterization / 43 3.4.1 Materials and methods / 43 3.4.2 Results and discussion / 43 3.5 Conclusions / 44 3.6 References / 45 4 ALKALI FREE HYDROLYSIS

Results for sodium borohydride kinetic experiments 49

4.1 Introduction / 49 4.2 Experimental procedure / 51 4.3 Results and discussion / 52 4.3.1 Effect of H2O/NaBH4 (mol/mol) on H2 generation yield / 53 4.3.2 Effect of catalyst/NaBH4 (g/g) on H2 generation rate / 56 4.3.3 Effect of pressure on hydrogen generation yield and rate / 56 4.3.4 Effect of reactor bottom shape on hydrogen generation / 58 4.3.5 Effect of temperature on hydrogen generation rate / 60 4.3.6 Gravimetric and volumetric densities / 61 4.4 Reaction by-product characterization / 63 4.4.1 Materials and methods / 63 4.4.2 Results and discussion / 64 4.5 Conclusions / 64 4.6 References / 65 5 less POLAR ORGANIC POLYMERIC SOLUTIONS HYDROLYSIS

Results for sodium borohydride kinetic experiments 67

5.1 Introduction / 67 5.2 Experimental procedure / 69 5.3 Results and discussion / 69 5.4 Reaction by-product characterization / 72 5.5 Conclusions / 75 5.6 References / 76

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6 NICKEL BASED BIMETALLIC CATALYST CHARACTERIZATION Results after 200 reutilizations

77

6.1 Introduction / 77 6.2 Catalyst characterization / 78 6.2.1 Textural properties / 78 6.2.2 Morphology / 81 6.2.3 XPS analysis / 84 6.3 Catalyst activity analysis after 200 reutilizations / 88 6.4 Conclusions / 89 6.5 References / 90 7 CONCLUSION 93 7.1 Conclusions / 93 7.2 Suggestions for future work / 96

xvii

LIST OF FIGURES

CHAPTER 1

Figure 1.1. Volumetric and gravimetric capacities for several possible H2 storage technologies. [1.3] / 3

Figure 1.2. Status of hydrogen storage technologies. Capacities for both materials-only basis and total system are shown [1.9]. / 5

Figure 1.3. The first fuel cell, by William Grove (1839). / 7

Figure 1.4. (A) Schematic of a typical H2/O2 PEMFC [1.13]. (B) Nafion® Polymer Electrolyte Membrane Structure and Characteristics. / 9

Figure 1.5. Front view and top view of the “MicroBoro Bus” (A – connection to the H2 generator and storage tank; B- gas-washing bottle; C- PEMFC single cell). / 10

CHAPTER 2

Figure 2.1. Photograph of the nickel based bimetallic catalyst in the form of a finely divided black powder. / 16

Figure 2.2. Photo Scanning electron microscope view of the synthesized catalyst powder (a) and associated elemental analysis EDAX (b). / 17

Figure 2.3. Nitrogen adsorption/desorption isotherms. / 18

Figure 2.4. Photograph of the main batch reactor with the two separate filling pieces used to change the internal free volume for NaBH4 hydrolysis reactions. / 19

Figure 2.5. Schematic view of the reactors inside: reactor LG – flat bottom, and reactors MR and SR – conical bottom, with mention of the internal available free volume. / 20

Figure 2.6. Global picture of the experimental setup used for the kinetics studies of hydrogen production via catalytic hydrolysis of sodium borohydride, under pressure. (1) batch reactor; (2a-2b) data acquisition system; (3) “MicroBoro bus” with a PEM fuel cell; (4) thermostatic bath and (5) refrigeration unit. / 21

Figure 2.7. Survey picture of the: (a) main reaction vessel / LR; (b) accessories fixed on the outside lid of the reactor: 1- reactor inlet bore valve, 2- syringe, 3- pressure probe, 4- Bourdon manometer and 5- reactor exhaustion needle valve; (c) ascendant view of the inside lid of the reactor: 1- needle of the syringe, 2- “T.Bottom” k thermocouple and 3- “T.Top” k thermocouple; (d) top view of the inside lid of the reactor showing the left ‘holes’ after connecting the some accessories; (e) syringe with needle (150 mm length). / 22

CHAPTER 3

Figure 3.1. Hydrogen generation plot with a NaBH4 concentration of 10%, an inhibitor concentration of 3%, at different temperatures. The reactions, for 20 cm3 of reactant solution, were performed in the MR batch reactor (369 cm3) with a proportion of Ru-Ni based catalyst/NaBH4: 0.4 g/g. / 35

Figure 3.2. Hydrogen generation plot with an inhibitor concentration of 3%, at 27 ºC, and with different NaBH4 concentrations. The reactions where performed in LR batch reactor (646 cm3) with a proportion of Ru-Ni based catalyst/NaBH4: 0.4 g/g. / 36

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Figure 3.3. Hydrogen generation with a NaBH4 concentration of 10%, at 25 ºC, with different NaOH concentrations. The reactions, for 10 cm3 of reactant solution, were performed in LR batch reactor (646 cm3) with a proportion of Ru-Ni based catalyst/NaBH4: 0.4 g/g. / 37

Figure 3.4. Hydrogen generation with a NaBH4 concentration of 10 wt%, an inhibitor concentration of 7wt%, at 45 ºC, for the three design reactors: LR (646 cm3), MR (369 cm3) and SR (229 cm3). The reactions, for 20 cm3 of reactant solution, were performed with a proportion of Ru-Ni based catalyst/NaBH4: 0.4 g/g. / 38

Figure 3.5. Hydrogen generation plots with a NaBH4 concentration of 10%, an inhibitor concentration of 7%, at ≈ 26 ºC, for two batch reactors with different bottom shape. The reactions, for 10 cm3 of reactant solution, were performed with a proportion of Ru-Ni based catalyst/NaBH4: 0.4 g/g. / 38

Figure 3.6. Five successive loadings of the reactant solution: 10 wt.% NaBH4, 7 wt.% NaOH, 83 wt.% H2O, with Ni-based/NaBH4: 0.4g/g, performed in the batch reactor LR (646 cm3), showing the temperature profile inside the reactor at two specific points (bottom and top of reactor). / 39

Figure 3.7. Hydrogen generation in batch reactor LR (646 cm3) of five successive loadings of the reactant solution: 10 wt.% NaBH4, 7 wt.% NaOH, 83 wt.% H2O, with Ru-Ni based catalyst/NaBH4: 0.4g/g. / 40

Figure 3.8. Hydrogen generation in batch reactor MR (369 cm3) of seven successive loadings of the reactant solution: 10 wt.% NaBH4, 7 wt.% NaOH, 83 wt.% H2O, with Ru-Ni based catalyst/NaBH4: 0.4g/g. / 41

Figure 3.9. Two images of reaction “gaseous” by-product, after seven successive loadings of reactant solution (10 wt.% NaBH4, 7 wt.% NaOH, 83 wt.% H2O). The H2 release shows the gas capability to agitate the nickel-based powdered catalyst presented at the reactor liquid phase. / 41

Figure 3.10. Hydrogen generation in batch reactor LR (646 cm3) of eight successive loadings of the reactant solution: 10 wt.% NaBH4, 7 wt.% NaOH, 83 wt.% H2O, with Ru-Ni based catalyst/NaBH4: 0.4g/g (the 8th loading was performed with magnetic stirring). / 42

Figure 3.11. Crystals pictures of the by-product classic hydrolysis of sodium borohydride, obtained by slow evaporation of a water solution at room temperature (≈ 25 ºC). / 43

Figure 3.12. Views of crystal structure of sodium metaborate dehydrate showing stacking layers formed by BO3 triangles and by Na and water molecules: (a) view along the a-axis and (b) view along the c-axis. Na in violet, O in red, B in pink and H in white. / 44

CHAPTER 4

Figure 4.1. Pictures showing the preparation of the reactant blend (catalyst plus NaBH4, both solids) to use in alkali free hydrolysis experiments. From left to right: Ru-Ni based catalyst, anhydrous sodium borohydride and the bottom of MR reactor with the mixture (stored in bottom conical shape). / 52

Figure 4.2. Hydrogen generation in the flat bottom batch reactor LR (646 cm3) for experiments with Ru – Ni based catalyst/NaBH4: 0.4 g/g and H2O/NaBH4: 2-8 mol/mol. / 53

Figure 4.3. Hydrogen generation in the batch reactor LR (646 cm3) with Ni-based/NaBH4: 0.4 g/g (27 and 28 times reused) and H2O/NaBH4: 2 mol/mol, after two successive loadings of pure water (of the same molar quantity). / 55

Figure 4.4. Hydrogen yield as a function of added water, H2O/NaBH4 (mol/mol), in batch reactor LG (646 cm3). / 55

Figure 4.5. Influence of the amount of catalyst/NaBH4 (g/g) on hydrogen generation. / 56

Figure 4.6. Hydrogen generation in the studied three batch reactors - LR (646 cm3), MR (369 cm3) and SR (229 cm3), with Ni-Ru based catalyst/NaBH4: 0.4 g/g (111, 150 and 151 times reused, respectively for LR, MR and SR) and H2O/NaBH4: 4 mol/mol. / 57

Figure 4.7. Influence of pressure on H2 yield (Experiments done in three batch reactors at the temperature range of 289-295 K, with catalyst reused ≈150 times). / 57

xix

Figure 4.8. Influence of reactor bottom shape on H2 yield, rate and lag time (experiments performed in batch reactors LR (646 cm3) and MR (369 cm3), with flat and conical bottom shapes, respectively; at temperature range of 289-295 K, with catalyst reused ≈150 times). / 58

Figure 4.9. Photographs of MR reactor (with conical bottom shape) after an alkali free hydrolysis of NaBH4 experiment. In the right, a view of the solid mixture of the by-product plus the catalyst. / 59

Figure 4.10. Temperature of the hydrolyzed NaBH4 as a function of time by different pressures. / 60

Figure 4.11. Influence of temperature on hydrogen generation rate and yield. / 61

Figure 4.12. View along the c-axis of the crystal structure of sodium metaborate hydrate: (a) 4H2O, in MR at 0.69 MPa and (b) 2H2O, in SR at 1.26 MPa. The element color code, from black to light grey, is B, O, Na and H. / 64

CHAPTER 5

Figure 5.1 Hydrogen generation by a single loading of 20 cm3 of reactant solution, at 45 ºC, with Ni-Ru based catalyst/NaBH4: 0.4 g/g: (a) MR batch reactor [0.369 L] and (b) SR batch reactor [0.229 L]. / 70

Figure 5.2. View along the a-axis of the crystal structure of sodium borate anhydrous, showing the BO4 tetrahedral arrangement and the Na bound network. Na in violet, O in red and B in pink. / 73

Figure 5.3. View along the a-axis of the crystal structure of sodium borate anhydrous, showing the BO3 triangular and the BO4 tetrahedral arrangements sharing oxygen atoms and the Na bound network. Na in violet, O in red and B in pink. / 73

Figure 5.4. Gravimetric H2 storage density (reactants basis) for the three systems studied in this chapter. / 75

CHAPTER 6

Figure 6.1. Nitrogen isotherms of powder Ru-Ni based catalyst 200 times reused. / 80

Figure 6.2. Pore size distributions of powder Ru-Ni based catalyst 200 times reused. / 80

Figure 6.3. SEM micrographs of powder Ru-Ni based catalyst, 200 times reused. Magnifications between 200 and 100 000X, increasing from (a) to (e). / 82

Figure 6.4. SEM image coupled with EDS spectrum of Ru-Ni based catalyst 200 times reused and a magnification of 200X. / 83

Figure 6.5. SEM image coupled with EDS spectrum of Ru-Ni based catalyst 200 times reused and a magnification of 1500X. / 84

Figure 6.6. SEM image coupled with EDS spectrum, at three different selected areas, of Ru-Ni based catalyst 200 times reused and a magnification of 10 000X. / 85

Figure 6.7. Photograph of Ru-Ni based catalyst 200 times reused pellet, with 10 mm in diameter, for use in XPS analysis. / 86

Figure 6.8. A survey XPS spectrum of Ru-Ni based catalyst 200 times reused. / 86

Figure 6.9. XPS spectra for Ru-Ni based powdered catalyst 200 times reused. / 87

Figure 6.10. Hydrogen generation plots with a NaBH4 concentration of 10%, an inhibitor concentration of 7%, for two different stages of catalyst reutilization (28 times reused and 200 times reused). The reactions, for 10 cm3 of reactant solution, were performed in LR batch reactor (646 cm3) with a proportion of Ru-Ni based catalyst/NaBH4: 0.4 g/g, at room temperature of ≈ 25 ºC. / 89

xxi

LIST OF TABLES

CHAPTER 1

Table 1.1 – Combustion enthalpies for some common fuels [1.2]. / 2

Table 1.2 – FreedomCar requirements stated by the US DOE [1.4]. / 6

CHAPTER 2

Table 2.1 – Textural properties of catalyst powder used in this work before use. / 18

CHAPTER 3

Table 3.1 – Excess hydration factor, x, of aqueous solutions of sodium borohydride. / 28

Table 3.2 – Comparison of gravimetric efficiencies of various reactant preparations. / 29

CHAPTER 4

Table 4.1 - The FreedomCAR targets publish by US Department Of Energy (DOE). / 62

Table 4.2 - Influence of Ni-Ru based catalyst/NaBH4 on gravimetric hydrogen density and hydrogen yield for H2O/NaBH4: 4 mol/mol. Prediction of the volumetric hydrogen density*. / 63

CHAPTER 5

Table 5.1 – Hydrogen yield for the studied reactant solutions in the two batch reactors and prevision of respective H2 solubility effects, at 45 ºC. / 71

Table 5.2 – Values of conductivity and pH of the studied reactant solutions, before and after reaction completion. / 72

CHAPTER 6

Table 6.1 - Textural properties of powder Ni-Ru based catalyst 200 times reused sample. / 80

Table 6.2 - Specific conditions for XPS analysis of Ni-Ru based catalyst 200 times reused. / 86

Table 6.3 - Results of XPS analysis of Ni-Ru based catalyst 200 times reused. / 88

Table 6.4 - Results for yield, lag time and dtdP/ slope in hydrogen generation of 10 mL of 10wt% NaBH4 and 7wt% NaOH solution, at two different stages of Ni-Ru based catalyst reutilization (for 28 and 200 times reused), in batch reactor LR (646 cm3). / 89

xxiii

NOMENCLATURE

yield, yield = n(H2)exp / n(H2)theoretical (n= number of moles)

x , excess hydration factor

Vµ , micropore volume by t-method;

Vp , total pore volume, for P/P0=0.986.

V , molar volume of the liquid

t1/2 , the half-life time, min

T , absolute temperature, K

Sext , (mesopore + macropore) surface areas by t-method;

SBET ,BET surface area;

r , the pore radius

0/ PP , relative pressure

Greek letters

φ , contact angle

γ , its surface tension

1

CHAPTER 1

Introduction

In this introductory chapter the topic and main objectives of this dissertation, entitled “Catalytic generation and storage of hydrogen from hydrolysis of sodium borohydride solutions under pressure – application in a hydrogen/oxygen fuel cell” are presented. A general introduction to hydrogen gas as an energy carrier, to sodium borohydride as hydrogen carrier and to polymer electrolyte membrane fuel cells is carried out.

1.1 GENERAL INTRODUCTION

This dissertation investigates a potential improvement to hydrogen (H2) generation and

simultaneous storage, by catalytic hydrolysis of sodium borohydride (NaBH4) under

pressure, for supply fuel cells of the type PEM – Polymer Electrolyte Membrane.

Due to the accelerate decrease of fossil fuels resources and the continuous growing of

energy demand, the development of alternative and clean renewable energy carriers is of

the utmost importance. In fact, the world may be even closer to running out of oil than

usually admitted [1.1].

The transportation sector and other portable applications are areas where an alternative

energy source may have a particularly great impact. Thus, employment of a concentrated

energy (electricity) carrier is of vital interest and priority. In this context, hydrogen (H2) is

presented as an environmental friendly energy vector, since it may serve as an intermediate

through which a primary energy source (for example, solar) can be effectively transmitted

and consumed.

1.1.1 Hydrogen as energy carrier

The hydrogen is the simplest and the most abundant atom both on the Universe and in

Earth. The supreme important characteristic of hydrogen gas, the lightest and smallest

molecule on our planet, is the fact that it has the highest energy to weight ratio, or

CHAPTER 1 Introduction

2

gravimetric energy density, of the energy sources known today. Table 1.1, shows the

combustion enthalpies for some of the most common used fuels [1.2].

Table 1.1 – Combustion enthalpies for some common fuels [1.2].

fuel reaction energy/mole energy/kg Ethanol C2H5OH + 3O2 → 2CO2 + 3H2O 1367 kJ 2.97×104 kJ Coal C + O2 → CO2 393 kJ 3.27×104 kJ Petroleum 2C8H18 + 25O2 → 16CO2 + 18H2O 5452 kJ 4.47×104 kJ Natural gas CH4 + 2O2 → CO2 + 2H2O 883 kJ 5.50×104 kJ Hydrogen 2H5 + O2 → 2H2O 247 kJ 12.24×104 kJ

These enthalpic values reflect ideal (100%) combustion at 25 ºC. * Octane is only one of the many hydrocarbons in petroleum. º Methane is the primary constituent (>90%) of natural gas.

As can be seen from the previous table, H2 gravimetric storage density is the highest,

12.24×104 kJ, almost three times more than the average value of the others presented fuels.

Additionally, hydrogen as a fuel offers a number of attractive advantages, namely [1.2]:

i) minimal pollution characteristics;

ii) mass transportability comparable to that of petroleum;

iii) combustibility in internal combustion engines (Carnot cycle devices);

iv) feasibility of electrochemical combination with an oxidant in fuel cells (free energy

type devices).

Unfortunately, large quantities of molecular hydrogen do not exist in a readily usable form,

so it must be synthesized from other energy sources, and then used to transfer energy to

another use. For this intrinsic fact, clean and efficient hydrogen production and storage

methods still remain a great challenge.

Although the viability of hydrogen combustion, as illustrated in Table 1.1, fuel cells

utilization of H2 is potentially even more attractive. In truth, hydrogen can be used directly

in a PEMFC to generate electricity, producing water as a by-product. Since a PEMFC

operates at lower temperature and offers a higher power density than most fuel cells, it has

been widely accepted for use with automobiles and other portable applications.

Additionally, fuel cells are noiseless and have no moving parts. However, PEM fuel cell is

not free from problems mostly because it requires pure hydrogen. In Section 1.2, below, a

more detailed explanation of PEMFC is provided.

A system for production and distribution of H2 will most certainly be dependent on the

type of hydrogen storage used. Indeed, hydrogen has a very low volumetric energy density,

so it is very difficult to store a sufficient amount of hydrogen in a small and light enough


3

vessel. In fact, the challenge of hydrogen storage has proved to be the greatest obstacle

preventing hydrogen from replacing fossil fuels. Hence, a safe, efficient and economical

method of storing hydrogen must be available to implement a hydrogen economy based on

renewable resources.

There are presently four main approaches to store and deliver pure H2 gas at the point of

use: compressed gas, cryogenic liquid, reversible sorption/desorption, and chemical

storage. Each one of these technologies has practical challenges associated with its use.

Gravimetric and volumetric capacities of the storage system are the two primary screening

criteria. The gravimetric capacity or specific energy is a measure of the usable energy per

kilogram of system mass. In practical terms, this quantity is usually expressed as a weight

percentage of hydrogen. Volumetric capacity or energy density is the useful energy per

litre of system volume. Hydrogen poses unique storage challenges, because it is the lightest

and least dense gas, with only 8.98x10-5 kg/L at STP conditions. The current gravimetric

and volumetric efficiencies for the four pure hydrogen storage technologies are compared

in Figure 1.1 [1.3]. FreedomCAR targets published by US Department Of Energy (DOE)

[1.4] for automotive hydrogen storage systems in 2010 and 2015, are also shown.

Figure 1.1. Volumetric and gravimetric capacities for several possible H2 storage technologies. [1.3]

Regarding to the information depicted in Fig.1.1, an alternative to a fossil fuel or metal

complex alloy (low H2 density) based hydrogen delivery system, can be offered by

inorganic hydride salts – chemical hydrides, and may be utilized as a primary hydrogen

storage medium.


4

1.1.2 Chemical hydrides as hydrogen carrier: the particular case of NaBH4

Chemical hydrides are materials that produce hydrogen on demand through a chemical

reaction with water; and generally, they exhibit greater gravimetric energy densities and

are stable during long periods of storage without usage. Among the chemical hydrides,

sodium borohydride (NaBH4) is an example of a chemical hydride that has the advantage

of storing hydrogen in a stable and safe solution. Its hydrogen content is of 10.6 wt%, what

makes itself one of the highest hydrogen containing compounds.

Sodium borohydride reacts with water to generate molecular hydrogen according to the

hydrolysis reaction shown in equation (1.1):

NaBH4 + (2+x) H2O → NaBO2.xH2O + 4 H2 + heat. (1.1)

The standard state enthalpy change of the above reaction at 25 ºC is -217 kJ and is

therefore exothermic [1.5]. Unluckily, the rate of hydrogen release during the hydrolysis

reaction is low due to the increase of the solution pH as the basic metaborate (NaBO2) by-

product is formed. Schlesinger et al. [1.5] recognized the particular striking catalytic effect

of certain transition metals and their salts on sodium borohydride hydrolysis rate.

Regarding to equation (1.1), ideal hydrolysis is attained for 0=x [1.5], but in practice

excess of water is required accounting for the fact that the solid by-product (NaBO2.xH2O)

can exist with varying degrees of hydration [1.6]. Lyttle et al. [1.7] observed that

commercial borohydride deteriorates rapidly in aqueous solution, but is fairly stable in

basic solutions and furthermore the higher the pH the greater the stability. This finding

turned the solution pH as the limiting parameter of reaction (1.1) and forced researchers to

find a practical and controlled way of producing hydrogen by using catalysts or small

amounts of acid. Hence, heterogeneous catalysis offers a number of advantages:

independence of solution pH over a wide range, controllable hydrolysis rate and the reuse

of the catalyst.

In fact, hydrogen storage density of sodium borohydride can be calculated in several

ways, depending on the compound on which the weight density of hydrogen is based on.

For instance, the hydrogen storage capacity of sodium borohydride is 10.92 wt% by weight

and can be calculated by dividing the molar mass of hydrogen produced by the total molar

mass of NaBH4 plus the molar mass of the stoichiometric amount of water needed for

reaction (ideal hydrolysis, 0=x ). As mentioned before, a PEMFC produces water as a by-

product. If this water is recycled to produce hydrogen from NaBH4, the storage capacity


5

increases to 21.3 wt% [1.8]. However, one important factor to regard is that the by-product

of the chemical reaction (1.1) is still carried on-board. Hence, it must be included in the

storage density calculations. If so, the H2 storage density drops from 21.3 wt% to 12.2

wt%. As previously stated, the solid by-product, NaBO2, can exist with varying degrees of

hydration (NaBO2.xH2O), such as the tetrahydrate metaborate, NaBO2.4H2O. In this case,

the hydrogen storage density decreases to a level ≈ 4 wt%.

The US Department Of Energy (DOE) has published FreedomCAR requirements for

automotive hydrogen storage systems [1.4], probably the most stringent targets yet

articulated for hydrogen storage systems. These specifications are based on system mass

and volume, i.e., not only the storage material itself (materials-only basis) but also the

reactors, tanks, valves, and all supplementary equipment, which usually are call hardware.

In Figure 1.2, below, emphasis is given to chemical hydrides as materials with effective

high H2 storage capacities. The position of the several chemical hydrides shown in that plot

is calculated for materials-only basis.

NaBH4

NaBH4

NaBH4

Figure 1.2. Status of hydrogen storage technologies. Capacities for both materials-only basis and total system are shown [1.9].

As illustrate in the previous figure, chemical bonding of hydrogen is the only available

storage technique that can produce energy densities approaching to the FreedomCAR

targets. The potential of sodium borohydride system to meet these targets on a reactant-


6

only basis is highlighted and can be evaluated using the stoichiometry of equation (1.1). As

stated before, for the case of ideal hydrolysis (x = 0), the hydrogen storage capacity is

10.92 wt% or 0.1092 kg H2/kg reactants. Therefore, the hydrolysis of NaBH4 has the

potential to meet FreedomCar 2015 requirements (0.09 kg H2/kg system) for specific

energy if x is close to zero.

The mass that can be allocated for the hardware components can be estimate from the

FreedomCar targets and the calculated reactant-only specific energy. To compute the mass

that can be allocated for hardware, the FreedomCar 2015 target is compared to the

maximum theoretical specific energy, as shown below [1.6]:

reactants kg

hardware kg 2.0y

system kg 1

H kg 09.0

hardware kgy reactants kg 1

H kg 1092.0 22 =⇔=+

. (1.2)

As shown in (1.2), for NaBH4 with x=0, the allowable hardware mass is 0.2 kg of reactor

per kg of reactants, or 17% of the total system mass. As x increases, the allowable

component mass decreases until, at a value of x= 0.84, it is no longer possible to meet the

FreedomCar 2015 requirements. Hence, for an excess hydration of 2 the allowable reactor

mass is negative. This means that is not possible to design a reactor that meets the

FreedomCAR 2015 target if x=2. Table 1.2 shows the FreedomCar requirements for

specific energy density for the years 2010 and 2015, along with the allowable excess

hydration factor based on sodium borohydride hydrolysis [1.6]. Similar calculations can be

done for the other FreedomCar targets.

Table 1.2 – FreedomCar requirements stated by the US DOE [1.4].

Year 2010 2015 Usable specific energy from hydrogen Target (kWh/kg system) Target (kg H2/kg system)

2.0

0.06

3.0

0.09 Usable energy density from hydrogen Target (kWh/L system) Target (kg H2/m

3 system)

1.5 45

2.7 81

Allowable excess hydration factor based on NaBH4 hydrolysis x (reactants basis)

3.31

0.84

Allowed reactor mass ratio (reactor mass/mass of reactants) x = 0 x = 2

0.807 0.214

0.204

-0.190


7

We may conclude if x= 2 or even 4 the materials-only storage capacity is still promising

for some other niche of applications [1.10]. But, in practice to date the best systems require

that the NaBH4 be pre-dissolved in a large excess of water; the excess water and the mass

of the system equipment, significantly reduce the storage efficiency. However it is of great

interest to determine the amount of structural water that remains bound to the metaborate

product, for example, by X-Ray Diffractometry.

1.2 HYDROGEN/OXIGEN FUEL CELL

Fuel cells are not a new invention. In fact, the first recorded fuel cell utilized carbon and

nitric acid as fuel and was constructed in 1801 by Sir Humphrey Davy. The accreditation

for the invention of the fuel cell, however, is awarded to William Grove, who in 1839 used

hydrogen and oxygen to produce electricity. Grove discovered that by arranging two

platinum electrodes with one end of each immersed in a container of sulphuric acid and the

other ends separately sealed in containers of oxygen and hydrogen, a constant current

would flow between the electrodes. The sealed containers held water as well as the gases,

and he noted that the water level rose in both tubes as the current flowed. Grove realized

that combining several sets of these electrodes in a series circuit he might “effect the

decomposition of water by means of its composition”. Then after, he gave the name of a

‘gas battery’ – the first fuel cell (see Figure 1.3) [1.11].

Figure 1.3. The first fuel cell, by William Grove (1839).

The fuel cell invented by Grove stimulated research in nineteen century, but no practical

applications was developed. In twentieth century, during World War II, the british scientist


8

Francis Bacon designed the first alkaline fuel cell. In the 1960s, NASA showed some of

the potential applications for fuel cells by using them to provide power on space missions.

Currently, five types of fuel cells are receiving attention for energy production in

stationary or transportation applications. These are Alkaline Fuel Cells (AFC), Molten

Carbonate Fuel Cells (MCFC), Phosphoric Acid Fuel Cells (PAFC), Solid Oxide Fuel

Cells (SOFC), and Polymer Electrolyte Membrane Fuel Cells (PEMFC). All of these fuel

cell types have been demonstrated as stationary power plants with the advantage of

providing high quality on-site electrical power. The last decade has focused primarily on

using PEMFC’s for transportation purposes. Fuel cells also have the benefit of producing

constant amounts of electrical energy.

Hydrogen is the ideal fuel for fuel cells. In all types of fuel cells mention in the above

paragraph, hydrogen exhibits fast reaction kinetics and the only by product of the reaction

when oxygen is used as the oxidant is water.

1.2.1 Polymer Electrolyte Membrane Fuel Cell (PEMFC)

Polymer Electrolyte Membrane Fuel Cells (PEMFCs) have been the focus of a great deal

of research in the last few years due to the increasing demand for clean power that can be

supplied from a lightweight system. PEMFCs fall into this category due to their membrane

electrode assemblies (MEA). The MEA is sandwich composed of three different layers.

The first layer is the catalyst material, which consists of platinum particles (Pt) loaded into

a carbon cloth support. The next layer is the PEM. Although there are a few different types

of PEM materials, the industry standard is a copolymer of tetrafluoroethylene and

perfluoro[2-(fluorosulfonylethoxy)propylviny] ether. This material has a trade name,

Nafion®, and is made by DuPont [1.12]. The third layer is another Pt loaded catalyst layer

usually of the same composition as the first catalyst layer, depending on the fuel types. The

typical MEA is roughly 0.02’’ thick, and has negligible weight. Since the cell operates at

temperatures below 100 ºC, the shell of the fuel cell can be made out of lightweight

plastics. These characteristics make the PEMFC ideal for transportation power

applications. Figure 1.4 shows a typical PEMFC [1.13] and the chemical structure for

Nafion® Polymer Electrolyte Membrane Structure from DuPont.

The circled portion within Figure 1.2 shows the MEA.

The Polymer Electrolyte Membrane (PEM) is responsible for transporting the hydrogen

ion produced through catalysis at the anode to the cathode, where it recombines with


9

oxygen ions and electrons to form water. The transport of the hydrogen ion, a proton, is

made possible by the structure of Nafion®.

(A) (B)

Figure 1.4. (A) Schematic of a typical H2/O2 PEMFC [1.13]. (B) Nafion® Polymer Electrolyte Membrane Structure and Characteristics.

The blue section is a Teflon-like, fluorocarbon backbone that has hundreds of repeating

CF2-CF2-CF2- units. The brown section (vertical section of the molecular chain) is a side

chain that connects the backbone to the sulfonic acid ions. The SO3- anions are

permanently attached, but the H+ ions become mobile upon hydration of the membrane.

Thus, for a fuel cell to be functional, the fuel must be hydrated before entering the cell. For

hydrogen, this is done by running the gas through a washing bottle. In the case of methanol

or ethanol, the alcohol is in a water solution. The ion exchange capacity of Nafion® is

0.91meq/g (equivalent hydrogen mass per mass of dried Nafion®) [1.12].

In brief, the electrochemical reactions within an electrochemical cell are segregated to

two half cells, where an electrochemical reaction occurs at each half cell. Therefore, the

half cell reactions of a hydrogen/oxygen fuel cell, in an acidic electrolyte, are

Anode: H2 → 2H+ + 2e- (1.3)


10

Cathode: 2

1O2 + 2H+ + 2e-→ H2O

- (1.4)

Overall: H2 + 2

1O2 → H2O

- (1.5)

The future of hydrogen as an energy carrier will be determined by the oil market and the

price that society is willing to pay for an energy supply that is poor in carbon dioxide. The

costs of hydrogen production are, for the time being, prohibitive of any introduction into

the energy market. As soon as the world energy price permits, the hydrogen/electricity

combination will be open to an interesting future.

1.2.2 The “MicroBoro Bus” – a didactic demonstration prototype

The hydrogen generated in the experiments performed during this dissertation, was

supplied to a PEMFC single cell that was housed in a bus like shaped mobile platform,

used for didactic proposes – the “MicroBoro Bus” [1.14]. Figure 1.5 shows two pictures, a

side view and a top view, of the Microboro Bus. The single cell was fed with hydrogen

generated by the catalytic hydrolysis of NaBH4 experiments.

The developed low power PEM fuel cell uses an air breathing cathode and humidified

hydrogen produced by the borohydride reactor. Natural convection to air feed the cathode

is used when the platform is located on a stand and the energy conversion required is just

the one needed to make the wheels of the platform turn. Forced convection, by means of a

fan, supplies air to the cathode when the platform as a whole is programmed to move. The

necessary energy to drive the fan is provided by the fuel cell [1.14].

Figure 1.5. Front view and top view of the “MicroBoro Bus” (A – connection to the H2 generator and storage tank; B- gas-washing bottle; C- PEMFC single cell).

C

B

A


11

1.3 OBJECTIVES AND ORGANIZATION OF THE DISSERTATIO N

It is the objective of the present research to better understand the kinetic mechanism of

catalysed hydrolysis of sodium borohydride for producing molecular hydrogen at elevated

yields and rates, with high gravimetric and volumetric hydrogen densities. Hence, three

main different series of kinetic experiments of hydrolysis of sodium borohydride under

pressure, in the presence of reused nickel-ruthenium based catalyst, were carried out to

achieve this goal.

The present dissertation evaluates the following objectives:

1) Perform catalysed hydrolysis reactions of sodium borohydride in three different ways:

- by joining an alkali to sodium borohydride solutions – alkali hydrolysis;

- without joining an inhibitor – alkali free hydrolysis;

- by joining an organic polymer or surfactant – lPOPS hydrolysis,

to understand deeply the kinetic and chemistry of the hydrolysis and to investigate the

influence of pressure, temperature and reactant concentrations on the reactions rates and

yields;

2) Verify the influence of reactor bottom shape on hydrogen generation yields and rates;

3) Produce hydrogen by the concept of successive loadings of reactant solution;

4) Analyze the influence of magnetic stirring in refuelling hydrolysis;

5) Study the catalyst reutilization/durability and performance/capacity, after 200 times of

being used;

6) Characterise the catalyst, in terms textural properties, morphology and XPS analysis,

after a long period of reutilization.

The organization of the dissertation involves eight chapters concerned with:

Chapter 1, Introduction

Introduces the research topic and present the main objectives of this dissertation, entitled

“Catalytic generation and storage of hydrogen from hydrolysis of sodium borohydride

solutions under pressure – application in a hydrogen/oxygen fuel cell”. Short reviews of

hydrogen as a energy carrier, sodium borohydride as hydrogen carrier and PEM fuel cells

are presented.

Chapter 2, Experimental techniques and apparatus.

In this chapter the materials used in all the experimental studies are described, including

the nickel based bimetallic catalyst. For the latter, a textural and morphological

characterization is provided. The experimental rig, with special emphasis given to the

reactor design, and also a brief mention to reaction products analysis are offered.


12

Chapter 3, Alkali hydrolysis/Results for sodium borohydride kinetic experiments.

This chapter presents the results of alkali hydrolysis of sodium borohydride in the presence

of a reused nickel based bimetallic catalyst. The effects of temperature, NaBH4

concentration, NaOH concentration, and system pressure and reactor bottom shape on the

hydrogen generation rate are investigated. Particular importance is given to the effect of

successive loadings of reactant on catalyst activity, with and with out magnetic stirring.

Selected XRD analysis for the reaction by-product is also presented.

Chapter 4, Alkali free hydrolysis/Results for sodium borohydride kinetic experiments.

This chapter shows the results obtained on alkali free hydrolysis of sodium borohydride in

the presence of a reused nickel based bimetallic catalyst. A discussion on the stoichiometry

of hydrolysis, in terms of the number of added water molecules to solid NaBH4, is carried

out, based on the results of hydrogen generation rate and yields. Emphasis is given to

reactor bottom design effects on the hydrogen production. The reaction by-products are

analysed by XRD.

Chapter 5, less Polar Organic Polymeric Solutions for sodium borohydride kinetic

experiments.

This chapter illustrates the results obtained on lPOPS hydrolysis of sodium borohydride

plus small additions of an organic polymer and surfactant, in the presence of a reused

nickel based bimetallic catalyst. With the aim of improve the solubility of H2 in the

remaining solution, a discussion of the results of hydrogen generation rate and yields is

carried out. The reaction by-products are analysed by XRD.

Chapter 6, Nickel based bimetallic catalyst characterization / results after 200

reutilizations.

This chapter focuses on the characterization of the nickel based bimetallic catalyst after

200 reutilizations. Textural properties based on nitrogen adsorption isotherms; surface

morphology by scanning electron microscopy (SEM) coupled with EDS spectroscopy and

X-ray photoelectron spectroscopy (XPS) analysis, were used to characterized the powder

catalyst.

Chapter 7, Conclusion.

This chapter is devoted to conclusions and suggestions for future work. Conclusions drawn

from the results indicate that the reused ruthenium nickel based catalyst is capable of

catalyzing the sodium borohydride solutions at sufficient rates, in the three schemes

presented in this dissertation for comparison – alkali hydrolysis, alkali free hydrolysis and

lPOPS hydrolysis of sodium borohydride for hydrogen generation under pressure.


13

1.4 CONCLUSIONS

High performance fuel cells rely on a source of hydrogen for operation. Distribution of

hydrogen either in the gaseous or liquid state is energy intensive and requires extensive

infrastructure development to become a safe and reliable source of energy.

In this introductory chapter a short literature survey identified hydrogen gas as energy

carrier and chemical hydrides as high density hydrogen carriers. Special emphasis was

given to sodium borohydride which is stable in basic solution and can easily be

manipulated. The inherent capacity for on-site hydrogen generation further enhances the

projection for NaBH4 as hydrogen vector for supplying hydrogen/oxygen fuel cells. Hence,

a briefly outline of PEM fuel cells was also exposed.

Finally, the feasibility of borohydride source hydrogen in an operational PEM fuel cell

has been established. In truth, the hydrogen generated in the experiments performed during

this dissertation, was supplied to a PEM single fuel cell that was housed in a bus like

shaped mobile platform, used for didactic proposes – the “MicroBoro Bus” [1.14]

1.5 REFERENCES

[1.1] ‘Crucial data distorted as global oil runs dry’, The Guardian weekly, 13-19 Nov. 2009.

[1.2] ‘Catalytic generation of hydrogen from the hydrolysis of sodium borohydride. Application in a hydrogen/oxygen fuel cell. C.M. Kaufman, PhD Thesis, 1981.

[1.3] http://www1.eere.energy.gov/hydrogenandfuelcells/storage/tech_status.html.

[1.4] U.S. Department of Energy Hydrogen Program. Available from: http://www.hydrogen.energy.gov/ .

[1.5] H.I. Schlesinger, H.C. Brown, A.E. Finholt, J.R. Gilbreath, H.E. Hoeskstra, K. Hyde, Sodium borohydride, its hydrolysis and its use as a reducing agent and in the generation of hydrogen, J. Am. Chem. Soc. 75 (1953) 215-219.

[1.6] E. Marrero-Alfonso, J.R. Gray, T.A. Davis, M.A. Matthews, Hydrolysis of sodium borohydride with steam, Int. J. Hydrogen Energy 32 (2007) 4717-4722; Minimizing water utilization in hydrolysis of sodium borohydride: the role of sodium metaborate hydrates, Int. J. Hydrogen Energy 32 (2007) 4723-4730.

[1.7] D.A. Lyttle, E.H. Jensen, W.A. Struck, A simple volumetric assay for sodium borohydride, Analytical Chem. 24 (1952) 1843-1844.

[1.8] V.C.Y. Kong, F.R. Foulkes, D.W. Kirk, J.T. Hinatsu, Development of hydrogen storage for fuel cell generators. I – Hydrogen generation using hydrolysis hydrides, Int. J. Hydrogen Energy 124 (1999) 665-675.

[1.9] A. Züttel, P. Wenger, S. Rentsch, P. udan, Ph. Mauron, Ch. Emmenegger, LiBH4 a new hydrogen storage material. J. Power Sources 118 (2003) 1-7.


14

[1.10] U.B. Demirci, O. Akdim, P. Miele, Ten-year efforts and a no-go recommendation for sodium borohydride for on-board automotive hydrogen storage, Int. J. Hydrogen Energy 34 (2009) 2638-2645.

[1.11] W.R. Grove, Phi. Mag, 14 (1839) 127.

[1.12] DuPont, Nafion® perfluorinated polymer products, Product Information Guide, 2000.

[1.13] European Fuel Cell GMBH, 2005.

[1.14] Rangel C.M., Silva R.A., Pinto A.M.F.R. Fuel cells and on-demand hydrogen production: didactic demonstration prototype. Proceedings of the International Conference in Power Engineering and Electric Drives”, Eds. L.S. Martins and P. Santos, Setúbal, Portugal, September 12-14, 2007, paper 237.

15

CHAPTER 2

Experimental techniques and apparatus

In this chapter the materials used in all the experimental studies are described, including the nickel based bimetallic catalyst, the experimental setup with special emphasis to the reactor design, and also a brief mention of reaction products analysis.

2.1 INTRODUCTION

A detailed description of the materials used in the kinetic studies of the catalytic hydrolysis

of sodium borohydride is presented, both for the reactant solutions and for the nickel-based

bimetallic catalyst. The methods of preparation of the reactant solutions and/or pre-

treatment of the catalyst, the experimental rig design and detailed experimental plan for

three different kinds of kinetics studies: alkali hydrolysis, alkali free hydrolysis and less

Polar Organic Polymeric Solutions (lPOPS) are presented. The techniques used for

reaction products analysis: pure molecular hydrogen and sodium borates by-product are

also mentioned.

2.2 MATERIALS

2.2.1 Sodium borohydride

Anhydrous sodium borohydride powder (96% purity) was provided by MERCK (No.:

1.06371.0100) and stored in a dessicator until use.

In the experiments with the fuel (NaBH4) in the liquid form, used in alkali hydrolysis

and in lPOPS hydrolysis, the reactant solutions were stabilized by the addition of small

quantities of sodium hydroxide pellets as a hydrolysis inhibitor. The sodium hydroxide

CHAPTER 2 Experimental techniques and apparatus

16

(98% purity) was supplied by EKA (No.: 1310.73.2). Deionised water was used to prepare

all the aqueous reactant solutions and, in all of them, the initial pH was approximately 14.

In the experiments with alkali free hydrolysis of NaBH4 for hydrogen generation, the

anhydrous sodium borohydride powder was used in the solid state.

2.2.2 Nickel based bimetallic catalyst

The catalyst used in all the experiments reported in this dissertation is a nickel based

bimetallic catalyst, synthesized at LNEG – Laboratório Nacional de Energia e Geologia,

Fuel Cells and Hydrogen Unit, Lisbon – Portugal.

Catalyst preparation

The catalyst, in the form of a finely divided black powder (see Figure 2.1), unsupported,

was prepared from a mixture of precursors in deionised water - impregnating small

quantities of ruthenium in the nickel salts (Riedel-de Haën) - by chemical reaction with 10

wt% stabilised borohydride solution (Rohm and Haas), as the reducing environment. When

the reduction was complete the catalyst was appropriately decanted, washed, filtered, dried

and heat-treated at 110ºC. The catalyst was kept in a dessicator until use.

Figure 2.1. Photograph of the nickel based bimetallic catalyst in the form of a finely divided black powder.

Catalyst samples were analyzed for textural properties. The determination of surface

areas (SBET), by N2 physisorption was undertaken at 77K, using a Quantachrome

Instruments Nova 4200e apparatus. Prior to the analysis, the samples (0.127 g) were

degassed at 160 ºC for 3 h. Pore size distributions were obtained from the desorption

branch of the isotherms using the Barrett, Joyner and Halenda (BJH) method. The

micropore volumes and mesopore surface areas were determined by the t-method.


17

Morphology and elemental composition analysis of the catalyst were done on a

Scanning Electron Microscopy (SEM) coupled with EDS unit FEI Quanta 400 FEG

ESEM/EDAX Genesis X4M operating at 15 kV in low vacuum mode (LVSEM for

uncoated non conductive sample). The powder sample was prepared by simple dispersion

over a double side adhesive carbon tape. X-Ray Photoelectron spectroscopy (XPS) of the

catalyst before use, after the powder sample was made into a pellet of 10 mm diameter,

was carried out on a VG Scientific ESCALAB 200 A spectrometer using Mg Kα (1,253.6

eV) as a radiation source. The photoelectrons were analysed at a takeoff angle of 0º.

Catalyst characterization before used

The catalyst used in this work is a powder containing Ni and Ru species with nanometric

particle size, as evident in Figure 2.2a). The amount of ruthenium is small and was not

detectable by EDAX, see Figure 2.2b). However, by XPS analysis less than 1 At% Ru was

indicated.

(a)

Cou

nts

(a.u

)

(b)

(a)

Energy (keV)

Figure 2.2. Photo Scanning electron microscope view of the synthesized catalyst powder (a) and associated elemental analysis EDAX (b).

The nitrogen (N2) adsorption and desorption data as a function of pressure (at liquid

nitrogen temperature) are displayed in Figure 2.3. The results shows that de N2 isotherm


18

shape is of type IV, validating the BJH method used for characterized the textural

properties of our powder catalyst. Total specific surface area, as determined by N2

physisorption, gave a value of 54 m2g-1 and negligible micropore volume. In Table 2.1 the

textural properties of Ni-Ru based catalyst before used are summarized.

0

20

40

60

80

100

120

140

160

0 0,2 0,4 0,6 0,8 1Relative Pressure (P/P0)

Vo

lum

e (c

m3 /g

)

Figure 2.3. Nitrogen adsorption/desorption isotherms.

Table 2.1 – Textural properties of catalyst powder used in this work before use.

Catalyst SBET / m2g-1 Vµ / cm3 g-1 Sext

/ m2 g-1 Vp / cm3 g-1

Before use 54 0 52 0.22 SBET = BET surface area; Vµ = micropore volume by t-method; Sext = (mesopore + macropore) surface areas by t-method; Vp = total pore volume

After some preliminary hydrolysis rate tests with different proportions of Ni-based

bimetallic catalyst/NaBH4, it was decided that the appropriate relation to use in most of the

experiments reported in this dissertation is Ni-based bimetallic catalyst/NaBH4: 0.4 g/g.

After the first utilizations, the consequent kinetic experiments were done with reused

catalyst. The procedure adopted for the reutilization of the catalyst was to recover it from

the remaining solution in the reactor, to wash it five times with distilled water, then carry

out sedimentation by gravity for complete physical separation from distilled water, and

finally, dry the catalyst in an oven at 60 ºC for four hours.


19

2.3 APPARATUS FOR KINETIC STUDIES

2.3.1 Reactor design

The hydrolysis reactions were carried out in batch reactors, especially designed for the

experiments reported in this dissertation, all of them performed at pressures up to 3MPa.

The main batch reactor has an internal volume of 646 cm3 and was made of stainless steel

AISI 316, with wall width equal to 10 mm. Special care was taken with the value of the

wall width during the reactor design because all the kinetic experiments were to be done at

high pressures. As it is well known, hydrogen dissolution and permeation can be

significant at high pressure through most of materials, including stainless steels. This may

affect the integrity of structural components used for H2 storage during reaction, since

hydrogen can embrittle metals [2.1]. In order to allow for changes in the internal volume of

the main reactor, another two separate filled pieces that can be introduced inside the main

vessel without gap were designed and constructed as well in stainless steel AISI 316. The

three possible final design combinations of interest result in three different batch reactors,

named LR, MR and SR, with internal free volumes of 646 cm3, 369 cm3 and 229 cm3,

respectively. Figure 2.4 shows a photograph of the main reaction container with the two

different separate filled pieces.

Figure 2.4. Photograph of the main batch reactor with the two separate filling pieces used to change the internal free volume for NaBH4 hydrolysis reactions.

In comparison with the reactor used on an earlier work [2.2], two major improvements

were made in the reactor design. The upgrading is related with the reactor sealing, i.e., an

axial installation of the O-ring for the vessel sealing instead of a radial one; and with the


20

possibility of following the reaction temperature at two different cottas inside the vessel by

using two k thermocouples positioned at two distinct points: one near its bottom (named

T.Bottom) and the other one very close to its top (named T.Top). The first one is used to

monitor the increase of temperature in the core of the exothermic catalytic reaction, and

the second one, simply measures the temperature of molecular hydrogen in the free volume

of the container opened for the gas.

With the objective of studying the possible effect of the reactor bottom shape on the

hydrogen generation rates - this constitutes one of the originalities of the present

dissertation, an inside flat bottom and a conical bottom geometry were designed, to enable

non-dispersible effects during the contact of the fine powder catalyst either with the

reactant solutions or with the solid NaBH4 plus the injected pure water, the latter in the

alkali free hydrolysis experiments. Figure 2.5 show the exposed view of the inside

designed reactors.

Internal volume 646 cm3

T. Bottom T. Top


T. Bottom T. Top


T. Top

Reactor LR Reactor MR Reactor SR

Figure 2.5. Schematic view of the reactors inside: reactor LG – flat bottom, and reactors MR and SR – conical bottom, with mention of the internal available free volume.


21

2.3.2 Experimental rig

An overview photograph of the experimental rig is offered in Figure 2.6.

Figure 2.6 Global picture of the experimental setup used for the kinetics studies of hydrogen production via catalytic hydrolysis of sodium borohydride, under pressure. (1) batch reactor; (2a-2b) data acquisition system; (3) “MicroBoro bus” with a PEM fuel cell; (4) thermostatic bath and (5) refrigeration unit.

The typical hydrogen generation experiments involve the preparation of the reacting

solution by adding the appropriate amount of NaBH4 to a certain volume of aqueous

solution of the inhibitor NaOH (with a chosen hydroxide concentration) in the case of the

alkali hydrolysis or, a certain molar quantity of pure water, in the case of the alkali free

hydrolysis. After perfectly sealing the reaction autoclave, an adequate volume of reacting

solution (or pure water) is rapidly injected into the reactor by means of a syringe with a

large needle length (150 mm), to ensure that the reacting solution (or distilled water) is

delivered very close to the nickel-based bimetallic powder catalyst. This catalyst has been

previously stored in the bottom of the reactor, in a proper quantity, measured in an

analytical balance.

The temperature of the reactor medium was monitored at two different points, as

mentioned above, and recorded simultaneously with a data acquisition system (see 2a and

2b in Fig.2.6), using Labview software. To monitor the rate of hydrogen generation, using

the same data acquisition system, the gas pressure inside the reactor was followed with an

appropriate pressure transducer calibrated from 0-40 bar gauge and also by a Bourdon

manometer (from 0-60 bar gauge), until attaining a constant pressure inside the reactor. An

overview of the instrumentation fixed on the cover of the reaction chamber is offered in

Figure 2.7.

3 4

1

5

2a

2b


22

(a) (b) (c) (d)

(e)

Figure 2.7. Survey picture of the: (a) main reaction vessel / LR; (b) accessories fixed on the outside lid of the reactor: 1- reactor inlet bore valve, 2- syringe, 3- pressure probe, 4- Bourdon manometer and 5- reactor exhaustion needle valve; (c) ascendant view of the inside lid of the reactor: 1- needle of the syringe, 2- “T.Bottom” k thermocouple and 3- “T.Top” k thermocouple; (d) top view of the inside lid of the reactor showing the left ‘holes’ after connecting the some accessories; (e) syringe with needle (150 mm length).

Almost all the experiments reported on this dissertation were done with one single

injection of reactant solution and without magnetic stirring.

A thermostatic bath, coupled with a refrigeration unit (see 4 and 5 in Fig.2.6), was

efficiently used to control the reaction temperature, at a desired value between 19.5 and

55 ºC, in almost all the kinetic studied hydrolysis. In addition, some experiments were

done without temperature control, i.e., in environmental conditions, in order to compare

the hydrogen generation rates.

It is important to notice that prior to any of the studied hydrolysis, the atmospheric air

inside the reactor was not purged and no vacuum was done. So, the final gas predicted

pressure, assuming 100% conversion efficiency and by applying the ideal gas law, was

corrected by the addition of the corresponding air pressure (of the initial moles of air inside

the free volume of the reactor chamber).

2.4 GENERAL PLAN FOR NaBH4 HYDROLYSIS EXPERIMENTS

2.4.1 Alkali hydrolysis

Alkali catalytic hydrolysis of sodium borohydride for hydrogen generation under pressure

comprises a series of experiments done in the three batch reactors (LR, MR and SM), in

the presence of the powder reused nickel-based bimetallic catalyst, in a proportion of

catalyst/NaBH4: 0.4 g/g. The fuel, in the liquid form, was prepared with different weight

concentrations of the chemical hydride and inhibitor.

2

5 3 2

3

1

1

4


23

Briefly, the study developed under this research topic has the objective of revising the

effect of:

i) reaction temperature;

ii) NaBH4 concentration;

iii) NaOH concentration;

iv) pressure;

v) reactor bottom shape,

on hydrogen generation rates and yields.

Experimental tests were performed without magnetic stirring and, most often, at

controlled temperature (292-328 K).

2.4.1.1 Successive loadings of reactant solution

Also related with the item of alkali catalytic hydrolysis of sodium borohydride for

hydrogen production under pressure, a study of successive loadings of reactant solutions is

presented in this dissertation. The central target is to verify the activity of the Ni-based

bimetallic catalyst and put in evidence the capability of re-using a constant amount of

catalyst during successive loadings of fuel.

The experimental work comprises tests in two different batch reactors chosen, LR and

MR, in the presence of powder reused nickel-based bimetallic catalyst, in a proportion of

catalyst/NaBH4: 0.4 g/g. The tests were performed under uncontrolled ambient conditions

and without stirring.

In addition, and with the purpose of verifying the influence of agitation on hydrogen

reaction rate during successive loadings of fuel, a TeflonTM magnetic stirrer was placed

inside the reactor before one loading injection. Vigorous magnetic stirring was used to

promote complete mass transfer (to overcome diffusion limitations).

2.4.2 Alkali free hydrolysis

Alkali free catalytic hydrolysis of sodium borohydride for hydrogen generation under

pressure comprises also a series of experiments done in the three batch reactors (LR, MR

and SM), in the presence of powder reused nickel-based bimetallic catalyst. The reactant

blend was produced without adding NaOH aqueous solution to solid NaBH4, so this

research topic explores the alkali free hydrolysis of sodium borohydride for hydrogen

production. In this case, a proper quantity of powder catalyst and solid NaBH4, both

measured in an analytical balance, are mixed together in the solid state, and then stored in


24

the bottom of the reactor. After sealing perfectly the reaction vessel, a stoichiometric

amount of pure liquid water is rapidly injected into the reactor by means of a syringe.

The main inspiration behind developing this experimental item was to produce pure

hydrogen gas with very high gravimetric and volumetric densities.

In a few words, the alkali free hydrolysis experimental work aims at studying the

influence of:

i) H2O/NaBH4 (mol/mol) ratios between 2 and 8 mol;

ii) reactor pressure;

iii) reactor bottom shape; and

iv) catalyst/NaBH4 (g/g) ratios of 0.2 and 0.4 g,

on hydrogen generation rates and yields.

These experimental tests were performed without magnetic stirring and without

temperature control.

2.4.3 less Polar Organic Polymeric Solutions (lPOPS) hydrolysis

less Polar Organic Polymeric Solutions (lPOPS) hydrolysis of sodium borohydride for

hydrogen generation under pressure embraces a series of experimental work done in the

two batch reactors with conical bottom shape (MR and SM), in the presence of powder

nickel-based bimetallic catalyst, reused about 160 times. The reactant solutions were

produced by small additions of an organic polymer or surfactant to the stabilized aqueous

solution of NaBH4.

Also in this case, a proper quantity of powder reused Ni-Ru based catalyst, in the

proportion of catalyst/NaBH4 : 0.4 g/g, measured in an analytical balance, was stored in the

bottom of the reactor. After sealing perfectly the reaction vessel, 20 mL of the reactant

solution was rapidly injected into the reactor by means of a syringe. After reaction

completion, magnetic stirring inside de reactor was allowed for 30 minutes to promote

complete saturation of H2 with the remaining by-product solution. All the experimental

tests were performed with temperature control at 45 ºC.

The motivation behind developing the lPOPS was to increase the non-polar hydrogen-

electrolyte interactions and hence improve the affinity for H2 storage in the liquid phase by

solubility’s effects. Therefore, the lPOPS hydrolysis experiments aims at studying the

influence of:

i) the addition of 0.25 wt% CMC (Carboxil-Methyl-Cellulose) to the stabilized

solution of 10 wt% NaBH4 and 7 wt% NaOH;


25

ii) the addition of 0.25 wt% SDS (Sodium-Dodecyl-Sulphate) to the stabilized solution

of 10 wt% NaBH4 and 7 wt% NaOH; and

iii) the reactor pressure,

on hydrogen generation rates and yields, conducive to H2 solubility effects.

2.5 REACTION PRODUCTS ANALYSYS

2.5.1 Molecular hydrogen

The main product of the hydrolysis reactions mentioned above is hydrogen gas. A simple

verification that the gas feed to the PEMFC, when the reactor exhaustion needle valve is

opened, is molecular hydrogen, we forced the gas to go all the way through a gas-washing

bottle, filled with ultra pure water (conductivity of 1.48 µ S/cm at 25 ºC). After a period of

noteworthy H2 bubbling, the values of pH and conductivity of the remaining water inside

the gas-washing bottle were checked. In general, no major variance in the values of pH and

conductivity were found (typical final marks of 7.5 and 10 µ S/cm were found,

respectively, at 25 ºC). Although the proper analytical method to verify the composition of

the exhausted gas from the reaction vessel is the gas chromatography, it is reasonable to

assume that the great majority of the gas captured during the experimental work, which is

used to feed a PEM fuel cell hosted in a “Microboro bus” didactic prototype, is essentially

hydrogen.

2.5.2 Sodium borates by-products

The different course of hydrolysis reactions studied in this dissertation also produced

certain by-products, namely sodium borates (or oxygen-containing compounds of boron).

The by-product of the hydrogen generation reactions was dried by slow evaporation of

a water solution at the environmental conditions vivid in the laboratory. Suitable crystals

were then obtained and subsequently analyzed using one of the most common analytical

techniques to identify solid samples: the X-Ray Diffractometry (XRD). In fact, the X-ray

diffraction is primarily of value for the study of crystalline material. X rays are reflected

off the surfaces of crystals, and by studying the patterns of reflection as the crystalline

material is rotated in the path of the X rays, much information about the structure of the

material can be obtained.


26

The diffraction data were colleted at 293 K with a Gemini PX Ultra equipped with

MoKα radiation and with CuKα radiation (facility of IBMC - Instituto de Biologia

Molecular e Celular, University of Porto, Portugal).

2.6 CONCLUSIONS

In this chapter the materials used in all the experimental work were described, including

the nickel based bimetallic catalyst. The experimental setup gives special emphasis to the

reactor design, made of stainless steel AISI 316. To study the catalytic hydrolysis of

sodium borohydride under pressure, the O-ring used to seal the vessel was installed axially

instead of a radially. Moreover, to follow the reaction temperature at two different cottas

inside the vessel, two k thermocouples were positioned at distinct points: one near the

bottom and the other very close to the top. To study the influence of reactor bottom shape

on the hydrogen generation rates - this constitutes one of the originalities of the present

dissertation, two different bottom geometries were designed – one flat and the other

conical. The latter has the purpose of enabling non-dispersible effects during the contact of

the catalyst with the reactant solution.

The hydrolysis experiments were performed in the presence and in the absence of an

alkali, namely, alkali hydrolysis and alkali free hydrolysis of sodium borohydride,

respectively. Special attention is given to the refuelling experiments in order to validate the

catalyst activity. Also, a small addition of an organic polymer or surfactant was added to

the stabilized reactant solution with the objective of studying the H2 solubility effects in the

remaining solution inside the reactor after reaction completion.

Finally, suitable crystal of the by-products of the hydrolysis of sodium borohydride,

made by slow evaporation of a water content, where analyzed by XRD.

2.7 REFERENCES

[2.1] C. San Marchi, B.P. Somerday, S.L. Robinson, Permeability, solubility and diffusivity of hydrogen isotopes in stainless steels at high gas pressures, Int. J. Hydrogen Energy 32 (2007) 100-116. [2.2] A.M.F.R. Pinto, D.S. Falcão, R.A. Silva, C.M. Rangel, Hydrogen generation and storage from hydrolysis of sodium borohydride in batch reactors, Int. J. Hydrogen Energy 31 (2006) 1341-1347.

27

CHAPTER 3

Alkali hydrolysis Results for sodium borohydride kinetic experiments

This chapter presents the results of alkali hydrolysis of sodium borohydride in the presence of a reused nickel based bimetallic catalyst. The effects of temperature, NaBH4 concentration, NaOH concentration, system pressure and reactor bottom shape on the hydrogen generation rate are investigated. Particular importance is given to the effect of successive loadings of reactant on the catalyst activity. Selected XRD analysis for the reaction by-product is also presented.

3.1 INTRODUCTION

Sodium borohydride has been extensively studied as a hydrogen storage medium because it

is readily available, relatively inexpensive, and more stable than other chemical hydrides

(for instance, lithium or aluminium borohydrides) [3.1-3.7]. Almost all prior work uses an

excess of liquid water, with a catalyst or promoter to achieve high yields and rates of

hydrogen.

NaBH4 reacts with water to generate molecular hydrogen according to the hydrolysis

reaction shown in Eq.(3.1):


Ideal hydrolysis is attained for 0=x [3.1], but in practice excess of water is required

to pre-dissolve the hydride for storage or to keep the by-products in solution (accounting

for the fact that the solid by-product, hydrated metaborate (NaBO2.xH2O), can exist with

various degrees of hydration [3.8-3.10]).

CHAPTER 3 Alkali hydrolysis

28

Table 3.1 show values of x (excess hydration factor) corresponding to aqueous

solutions of sodium borohydride, ranging from 5 wt% to 35 wt% (approximately the

solubility limit of NaBH4), found in most hydrolysis schemes published in literature.

Table 3.1 – Excess hydration factor, x, of aqueous solutions of sodium borohydride.

NaBH4 weight, % 5 7.5 10 20 30 35 x 37.9 23.9 16.9 6.4 2.9 1.9

From the values in Table 3.1 it is clear that solubility considerations alone cause

chemical hydride systems to lose significant storage efficiency relative to the hydrogen

content on a materials only basis. Hydrolysis is typically conducted in the aqueous phase at

lower temperatures, and large quantities of excess water are required because of the low

solubility of both NaBH4 and the borate by-products in water [3.11]. In pure liquid water

the reaction generates basic species that are thermodynamically and kinetically metastable.

Metastability causes the reaction to cease at low yields of hydrogen even though the

reaction is strongly favoured thermodynamically [3.12-3.14]. The metastable species

require addition of mineral acid or use of a heterogeneous catalyst to drive the reaction to

completion. Furthermore, the solid products of the reaction are an unknown mixture of

hydrated sodium metaborates. Hydrated products capture water that is not reduced to

hydrogen, further decreasing the efficiency of the reaction.

Most systems that have been engineered to date utilize a route wherein both the

hydride reactant as well as the product is dissolved in excess water. Because the solubility

of the chemical hydride (55g NaBH4/100g H2O at 25 ºC) [3.15] is relatively low, and the

solubility of the oxides is even lower (28g NaBO2/100g H2O at 25 ºC) [3.7], such systems

inherently have a greatly reduced gravimetric efficiency (hydrogen storage capacity on a

materials-only basis) compared to the ideal.

Table 3.2, shows the effective value of x for different amounts of water, and the

corresponding hydrogen storage capacity on a materials-only basis. Ideal hydrolysis

requires no excess water. Complete dissolution of NaBH4 at 25 ºC requires extra water

corresponding to x=1.81. If sufficient water is added to keep the NaBO2 in solution, the

effective value of x is 11.04.


29

Table 3.2 – Comparison of gravimetric efficiencies of various reactant preparations.

Molar ratio, NaBH4/H2O

x H2 storage capacity,

wt% Ideal hydrolysis 1 / 2 0 10.92

Saturated NaBH4 solution, 25 ºC 1 / 3.81 1.81 7.57 NaBO2 solubility limit, 25 ºC 1 / 13.04 11.04 2.96

8.5 wt% NaBH4 plus 5 wt% NaOH 1 / 21.37 19.37 1.81

Furthermore, the addition of NaOH or other base to stabilize the solutions also affects

the hydrogen storage density. Shang et al. [3.11] found that the optimum concentration,

where the compounds are stable and dissolve in solution, is 8.5wt% NaBH4/5wt% NaOH

resulting in a theoretical gravimetric storage efficiency of only 1.81wt% H2 (x=19.37).

Clearly, addition of water to achieve solubility of the reactants and/or products drives the

storage efficiency down rapidly.

Almost all research to date is based on conducting the reaction in an aqueous mixture,

with large amounts of excess water. Despite favourable thermodynamics, as said before,

aqueous phase reactions with pure water give conversions of less than 5%.

It has been reported that the decrease in the initial rate of hydrogen and reaction yield

is due to an increase in the pH of the solution, caused by the formation of the strongly

basic metaborate ion [3.1]. This stabilization under high pH conditions has also been

exploited to prevent premature reaction.

Fundamental investigations on the hydrolysis of chemical hydrides, particularly

lithium and sodium borohydrides, were performed by Schlesinger et al. [3.1] in the early

1950s. They observed that the rate of the aqueous hydrolysis reaction decreased with the

formation of the basic sodium metaborate (BO2-) product, and concluded that the reaction

rate is dependent on both temperature and pH. Kreevoy et al. [3.16] later described this

dependence by the empirical rate law presented by Equation (3.2),

)92.1034.0()log( 2/1 −−= TpHt , (3.2)

where t1/2 is the half-life of self-hydrolysis of NaBH4 in minutes at a particular temperature

T in K and pH is the solution pH value (in the absence of catalyst). That is to say, alkaline

NaBH4 solution at pH 14 can be kept for 430 days from self-decaying at room temperature.

The reaction yield also depends heavily on the pH and temperature. Kojima et al. [3.7]

reported that at room temperature (20-23 ºC) the overall reaction conversion stabilizes at


30

7% as the basic borate by-product is formed. Moon et al. [3.17] investigated the pH and

temperature effects on the rate of decomposition of more concentrated NaBH4 solutions.

Therefore, previous studies have shown that the uncatalyzed aqueous hydrolysis

reaction of NaBH4 proceeds at a very slow rate. In more concentrated solutions, the rate

was diminished probably due to solubility and mobility inhibition by the NaBO2 product

[3.17]. In all solutions with inhibitor, the kinetics of H2 generation was initially linear, but

non-zero-order behaviour became evident after several hours. It was found that the

addition of alkali stabilizers such as NaBO2, KOH and NaOH depressed the hydrogen

evolution, a property utilized in many practical chemical hydride systems to promote stable

long term storage.

Indeed, to create a basic environment in which NaBH4 hydrolysis is stifled, simple

hydroxides such as NaOH and KOH should be added. However, the resulting alkaline

solution continues to undergo slow reaction to release H2. Several studies have looked at

the decomposition rate of these solutions at various NaOH and NaBH4 concentrations and

temperatures to gauge their shelf life. It is apparent that the rate of hydrolysis for dilute

solutions is much different than that of concentrated solutions and that temperature has a

dramatic effect on the stability of the solutions.

The concentration of NaBH4 in solution must be maximized to improve energy density

but low enough to keep the NaBO2 by-product in solution [3.11]. Minkina et al. [3.18]

investigated the effects of temperature and NaOH concentration on the degradation of

concentrated NaBH4 solutions. They suggest that above 30 ºC, at least 5wt% NaOH must

be added to slow the hydrolysis, which is consistent with Shang work who suggested

working on systems of 8.5 wt% NaBH4 and 5wt% NaOH solution at room temperature

[3.11].

In order to release the hydrogen from these basic stabilized solutions, metal or acid

catalysts are required [3.1]. Various studies have been performed on the catalyzed

hydrolysis reaction of sodium borohydride [3.5-3.7,3.19]. It is documented in the literature

[3.12, 3.14] that sodium borohydride undergoes hydrolysis in aqueous solution to give

boric acid in more acidic solutions (pH<9) and borate in more basic solutions (pH>9), as

shown in the following equations:

BH4- + H+ + 3H2O → B(OH)3- + 4H2 , pH<9 (3.3)

BH4- + 4H2O → B(OH)4- + 4H2 , pH>9 (3.4)


31

If sodium metaborate, a water soluble and environmentally benign solid, is assumed to

be the sole product, the reaction can be written simply as in Equation (3.1).

In fact, catalysts play such a vital role in hydrogen evolution from hydrolysis of

NaBH4, that a huge number of substances have been attempted to be efficacious in

increasing the rate of hydrogen evolution but never to yield in lowering the controllability

of hydrogen generation.

Several metal catalysts are effective reaction catalysts to enhance hydrolysis of the

alkaline sodium borohydride solution. Among them, the catalysts with precious metals

most used are: rhodium, platinum, ruthenium [3.5, 3.6, 3.20], platinum supported on

LiCoO2, CoO and TiO2 [3.7, 3.21, 3.22] or carbon [3.23], ruthenium supported on IRA-

400 anion resin [3.24] or alumina pellets [3.25], ruthenium nanoclusters [3.26], and Pt/Pd

on carbon nanotubes (CNT) paper [3.27].

The most used high-performance catalysts containing non-noble metals are: nickel

[3.1, 3.28], nickel and/or cobalt borides [3.12, 3.29, 3.30-3.37], cobalt boride supported on

nickel foam [3.38, 3.39] or carbon [3.40], cobalt boride amorphous alloy powder and Pd/C

[3.42], cobalt on γ -alumina [3.43], hydrogenphosphate stabilized nickel(0) nanoclusters

[3.44], Co-Mn-B supported on nickel foam [3.45], and Co-P catalyst [3.46,3.47]. A metal

alloy catalyst containing a less precious metal: Ru60Co20Fe20 supported on activated carbon

fibber, has been reported by Park et al. [3.48], who found high hydrogen release in the

reaction given by Eq.(3.1).

In the present work particularly interested is focused in the work published on Ru

catalysts, preferably unsupported, for results comparison. Elsewhere, a comparative study

of the catalysts mentioned in the previous paragraphs with the Ni-Ru based powdered

catalyst will be published (but that study is not included in this dissertation).

After this short review, in the following sections, the results obtained with the

ruthenium nickel based powdered catalyst, unsupported, in the study of the hydrogen

release from alkali hydrolysis of sodium borohydride (NaBH4) will be presented. Reports

on hydrogen rates and yields are given where a proportion of 0.4 g/g in catalyst/chemical

hydride was found suitable, after preliminary kinetic tests. The effects of NaBH4 and an

inhibitor (sodium hydroxide, NaOH) concentration, as well as the process temperature, on

the rate of hydrogen production were also analysed.


32

3.2 EXPERIMENTAL PROCEDURE

The alkali catalytic hydrolysis of sodium borohydride for hydrogen generation under

pressure comprises a series of experiments done in the three batch reactors LR, MR and

SR, with internal free volumes of 646 cm3, 369 cm3 and 229 cm3, respectively described in

the previous chapter.

The experimental conditions of the investigations done on the effects of reaction

temperature, NaBH4 concentration, NaOH concentration, pressure of system and reactor

bottom shape on hydrogen generation rates and yields, are given below, separately.

The experimental method used for the particular case study of successive loadings of

reactant solutions, to verify the activity of the Ni-based bimetallic catalyst in active

refuelling, is also specified.

The hydrogen yield in reaction (3.1) was calculated by the following equation

Hydrogen yield = n(H2)exp / n(H2)theoretical , (3.5)

where n(H2)exp is the number of moles (mol) of generated H2 and n(H2)theoretical is the

theoretical amount of generated H2 assuming 100% conversion of NaBH4 by applying the

ideal gas law to the final volume of gas inside the reactor.

3.2.1 Effect of reaction temperature on hydrogen generation rate

To investigate the effect of reaction temperature on hydrogen generation rate of the

catalytic hydrolysis of sodium borohydride, a series of kinetic experiments were carried

out in a extensive range of temperatures of: 19.5, 23, 30, 45 and 55 ºC, to satisfy part of

real environmental conditions throughout the year (very low temperatures were not

considered in this dissertation).

The hydrolysis reactions were carried out in the batch reactor MR (internal volume

equal to 369 cm3), using 20 cm3 of 10 wt% NaBH4 and 3 wt% NaOH solution

( 3g/cm 045.1=ρ ).

After a proper quantity of reused Ni-Ru based powdered catalyst (≈ 0.85 g) was stored

in the bottom of the reactor and the reaction vessel was perfectly sealed, the batch reactor

RM was placed in a thermostatic bath to reach and maintain constant the system

temperature at each of the values of the set given above. Then, 20 cm3 of the reactant


33

solution was rapidly injected and the inlet valve closed (see legend of Fig.2.5b), with the

reactor submersed in the water bath. The generation of hydrogen was followed using the

data acquisition system until a constant pressure inside the reactor is reached. No magnetic

stirring was used.

In the experiments reported in this section, the Ni-Ru based powdered catalyst was

reused between 157 and 161 times.

3.2.2 Effect of NaBH4 concentration on hydrogen generation rate

To explore the effect of sodium borohydride concentration on hydrogen generation rate of

the catalytic hydrolysis of sodium borohydride, a series of kinetic experiments was

conceived at seven different weight amounts of NaBH4: 5, 10, 15, 20, 25, 30 and 40 wt%,

for a constant concentration of an inhibitor (NaOH) of 3 wt%. All the hydrolysis reactions

were executed in the batch reactor LR (internal volume of 646 cm3), using 20 cm3 of

reactant solution, at constant temperature of 27 ºC. The experimental procedure on this

research item is similar to that described on the above section. No magnetic stirring was

used.

In the experiments run in this part of the analysis, the Ni-Ru based powdered catalyst

was reused between 35 and 41 times.

3.2.3 Effect of NaOH concentration on hydrogen generation rate

To examine the effect of the inhibitor sodium hydroxide concentration on the hydrogen

generation rate of the catalytic hydrolysis of sodium borohydride, a sequence of kinetic

experiments were conceived at six different weight amounts of NaOH: 1, 3, 7, 10, 20 and

30 wt%, for a constant concentration of NaBH4 of 10 wt%. All the hydrolysis reactions

were conducted in the batch reactor LR (internal free volume of 646 cm3), using 10 cm3 of

reactant solution, at constant temperature of 25 ºC. The experimental procedure of this

research topic is analogous to that described on 3.2.1. No magnetic stirring was used.

In the experiments reported in this part of the analysis, the Ni-Ru based powdered

catalyst was reused between 57 and 62 times.

3.2.4 Effect of pressure on hydrogen generation rate

To study the influence of pressure on the hydrogen generation rate of the catalytic

hydrolysis of sodium borohydride, three individual experiments were carried out in each


34

one of the designed reactors: LR (646 cm3), MR (369 cm3) and SR (229 cm3). A volume of

20 cm3 of 10 wt% NaBH4 and 7 wt% NaOH solution ( 3g/cm 085.1=ρ ) was injected; and

the reaction evolved at controllable temperature of 45 ºC, by immersing the reaction vessel

in the thermostatic bath. No agitation was present in the solution inside the reactors.

In this section, the Ni-Ru based powdered catalyst was reused 141 times, 154 times

and 170 times, respectively, for the experiments performed in reactors LR, MR and SM.

3.2.5 Effect of reactor bottom shape on hydrogen generation rate

To understand the influence of the reactor bottom shape on the hydrogen production rate -

this topic constitutes one of the originalities of the present dissertation, two different batch

reactors were used in the kinetic studies, both with approximately the same internal volume

(≈ 369 cm3) but with different bottom shapes: one has a flat bottom and the other a conical

bottom geometry. A volume of 10 cm3 of 10 wt% NaBH4 and 7 wt% NaOH solution

( 3g/cm 085.1=ρ ) was chosen; and the reaction advanced more or less at a constant room

temperature of ≈ 26 ºC. A proportion of Ni-Ru based catalyst/NaBH4: 0.4 g/g (≈ 0.43 g)

was found suitable for the experiments. No agitation was present in the solution inside the

reactors.

The Ru-Ni based catalyst at this section was virgin, i.e., 0 times reused!

3.2.6 Effect of successive loadings of reactant solution on catalyst activity

With the objective of studying the influence of successive loadings of reactant solution on

the catalyst activity, an experimental work was performed with several successive loadings

of reactant alkaline NaBH4 solution, with a concentration of 10 wt% NaBH4 and 7 wt%

NaOH by weight ( 3g/cm 085.1=ρ ), in two different batch reactors. Fuel loadings of

10 cm3 were established and a proportion of Ni-Ru based/NaBH4: 0.4g/g (≈ 0.43 g) was

found appropriate and used in all experiments.

The hydrogen yield in reaction (3.1) was calculated assuming 100 % conversion of

NaBH4 by applying the ideal gas law to the final volume of gas inside the reactor, with

x=2. Special care was taken to correct the free varying volume of gas inside the reactor

between each successive reactant loading.

Reactors LR (646 cm3) and MR (369 cm3) were preferred in this research topic to

perform the entire experimental work, at uncontrolled room temperature and without

magnetic stirring.


35

The Ni-Ru based powdered catalyst was reused between 2 and 6 times in the

experiments conducted in the MR batch reactor, and reused between 28 and 34 times in the

experiments performed in the LR reactor.

3.2.6.1 Effect of agitation on hydrogen reaction rate

With the intention of verifying the influence of agitation on hydrogen reaction rate during

successive loadings of fuel, a TeflonTM magnetic stirrer was placed inside the reactor LR

after the 7th successive loading and prior to the 8th fuel injection, in a run of experiments

with eight successive loadings. Vigorous magnetic stirring, set at a high level (level 8 of 10

of the stirrer), was allowed immediately after the 8th injection and the inlet reactor valve

closed. The generation of H2 was followed using the data acquisition system until reaching

a constant pressure inside the LR reactor. Inspection on the variation of system pressure

with time, during the 8th H2 generation, put forward the importance of considering

diffusion limitations in this particular subject of successive fuel loadings.

The Ni-Ru based powdered catalyst was reused between 226 and 233 times in these

experiments!

3.3 RESULTS AND DISCUSSION

3.3.1 Effect of reaction temperature on hydrogen generation rate

The influence of temperature on the velocity of hydrogen generation is put in evidence in

Figure 3.1.

0

5

10

15

20

0 500 1000 1500

time / s

Pre

ssu

re /

bar

19.5 ºC | yield=100%

23.0 ºC | yield=100%

30.0 ºC | yield=95%

45.0 ºC | yield=100%

55.0 ºC | yield=100%

theoretical yields

Figure 3.1. Hydrogen generation plot with a NaBH4 concentration of 10%, an inhibitor concentration of 3%, at different temperatures. The reactions, for 20 cm3 of reactant solution, were performed in the MR batch reactor (369 cm3) with a proportion of Ni-Ru based catalyst/NaBH4: 0.4 g/g.


36

As can be seen in the figure, the reaction rate is very sensitive to this variable. As

expected, the rate rises with the increase in temperature and the hydrogen pressure

demonstrates a near linear variation with reaction time. The influence of temperature is

clearly shown by the increasing slope values on the linear region of the plots, for

increasing values of the reaction temperature. The reaction rate remains practically

constant when, during the hydrolysis process, the NaBH4 concentration decreases due to

hydride and water consumption, demonstrating, under the experimental conditions studied

zero-order reaction as was also found by Kaufman and Sen [3.12] and Hua et al. [3.30].

The time needed for the reaction to begin is almost the same for al the reactions depicted in

Fig.3.1.

3.3.2 Effect of NaBH4 concentration on hydrogen generation rate

Figure 3.2, shows the hydrogen generation rate curves obtained with seven different

concentrations of NaBH4.

0

2

4

6

8

10

12

0 500 1000 1500time / s

Pre

ssu

re /

bar

5 wt% NaBH4 | yield=100%







Figure 3.2. Hydrogen generation plot with an inhibitor concentration of 3%, at 27 ºC, and with different NaBH4 concentrations. The reactions where performed in LR batch reactor (646 cm3) with a proportion of Ni-Ru based catalyst/NaBH4: 0.4 g/g.

Hydrogen generation rate increases with increasing the alkali concentrations up to 20

wt% in the alkali. Then, the reaction rate and hydrogen yield are both slightly lower for the

higher concentration of borohydride, above 20 wt%. This is due to the fact that, for these

levels of NaBH4 concentrations, as the hydrolysis reaction proceeds, the concentration of

the reaction product NaBO2 exceeds its solubility limit and precipitates out of solution, as


37

has been mentioned in Section 3.1. This fact may block catalysts sites, thereby affecting

the generation rate. This explanation was already proposed by Amendola et al. [3.5,3.6].

3.3.3 Effect of NaOH concentration on hydrogen generation rate

Figure 3.3 shows the dependence of the classic hydrolysis reaction rate on the NaOH

concentration.

0

1

2

3

4

5

0 100 200 300 400 500time / s

Pre

ssu

re /

ba

r

1 wt% NaOH | yield=100%






theoretical yields

Figure 3.3. Hydrogen generation with a NaBH4 concentration of 10%, at 25 ºC, with different NaOH concentrations. The reactions, for 10 cm3 of reactant solution, were performed in LR batch reactor (646 cm3) with a proportion of Ni-Ru based catalyst/NaBH4: 0.4 g/g.

The reaction rate is greatly enhanced by the increase of the NaOH concentration, as

can be see if Fig.3.3, with the highest value found for inhibitor concentration of 30 wt%.

This result was already reported by Hua et al. [3.30] who used a nickel boride catalyst.

3.3.4 Effect of pressure on hydrogen generation rate

The influence of the pressure in the hydrogen generation efficiency is put in evidence in

the plot of Figure 3.4.

In fact, for two of the experiments performed in this section, in reactors MR and SR,

an average yield of 92% was reached, i.e., the maximum hydrogen pressures reached were

substantially lower than the maximum predicted hydrogen pressure, assuming 100%

conversion, by applying the ideal gas law. Pinto et al. [3.33], who worked in a similar

experimental work, found the same behaviour in the H2 yield, and related this finding with

solubility effects in the liquid phase that remains inside the reactor. Thus, it seems possible


38

that increasing the gas pressure enhances strongly the H2 dissolution, forcing part of the

molecular H2 to be in the liquid. Consequently, H2 yields diminished.

0

5

10

15

20

25

30

0 100 200 300 400 500time / s

Pre

ssu

re /

ba

rLR (646 cm3) - flat bottom | yield=100%

MR (369 cm3) - conical bottom | yield=92%

SR (229 cm3) - conical bottom | yield=92%

Figure 3.4. Hydrogen generation with a NaBH4 concentration of 10 wt%, an inhibitor concentration of 7wt%, at 45 ºC, for the three design reactors: LR (646 cm3), MR (369 cm3) and SR (229 cm3). The reactions, for 20 cm3 of reactant solution, were performed with a proportion of Ni-Ru based catalyst/NaBH4: 0.4 g/g.

3.3.5 Effect of reactor bottom shape on hydrogen generation rate

Finally, the effect of reactor bottom shape on hydrogen generation is outlined in Figure

3.5. Definitely, as can be seen in the plot of Fig.3.5, the reactor bottom shape affects the

course of hydrogen generation.

Looking at the results illustrated in Fig.3.5, the conical bottom geometry adopted in

MR batch reactor increases the reaction rate and yield, in the absence of H2 lag time

(defined as the time required observing formation of H2).

0

2

4

6

8

10

0 200 400 600 800

time / s

Pre

ssur

e /

ba

r

MR-369 cm3, conical bottom | H2 yield=100% R-359 cm3, flat bottom | H2 yield=98%

Figure 3.5. Hydrogen generation plots with a NaBH4 concentration of 10%, an inhibitor concentration of 7%, at ≈ 26 ºC, for two batch reactors with different bottom shape. The reactions, for 10 cm3 of reactant solution, were performed with a proportion of Ni-Ru based catalyst/NaBH4: 0.4 g/g.


39

3.3.6 Effect of successive loadings of reactant solution on catalyst activity

Figure 3.6, shows the ''dP/dt behaviour of five 10 cm3 single successive loadings of

reactant solution, with concentration of 10 wt% NaBH4, 7 wt.% NaOH, 83 wt.% H2O, in

the batch reactor LR (646 cm3). It was found appropriate to show, in the same figure, the

variation of temperature inside the reactor that occurs during a single fuel loading. Similar

trends were detected in the experiments performed in the batch reactor MR.

As can be seen in the plot of Fig. 3.6, the rise of temperature at the bottom of the

reactor (T.Bottom) occurs during each individual fuel loading until the “plateau” is

reached, and this effect is more pronounced in the last loadings. When the temperature of

the system is not controlled, as is this case, the exothermic characteristics of reaction (3.1)

give rise to the medium gas temperature near the top of the reactor (positioned vertically).

This is favourable, due to the hydrolysis kinetic rates, because it leads to an increase of H2

pressure with smaller amount of the needed activation energy required by the (same

quantity of) catalyst, which is housed in the bottom of the reactor.

10 wt% NaBH4 and 7 wt% NaOH | five successive loadings

00,5

11,5

22,5

33,5

44,5

55,5

0 1000 2000 3000 4000 5000 6000

time / sec

Pre

ssu

re /

bar

20

25

30

35

40

45

Tem

per

atu

re /

ºC

T Bottom T Top

1st load 2nd load 3rd load4th load 5th load

Figure 3.6. Five successive loadings of the reactant solution: 10 wt.% NaBH4, 7 wt.% NaOH, 83 wt.% H2O, with Ni-Ru based/NaBH4: 0.4g/g, performed in the batch reactor LR (646 cm3), showing the temperature profile inside the reactor at two specific points (bottom and top of reactor).


40

The fluctuations detected in the reactor temperature along reaction (see Fig. 3.6), also

indicate at which rates the hydrogen is being generated, i.e., a smooth pattern of the

internal reactor temperature profile, both at the bottom and at its top, indicates a slowly

hydrolysis kinetics; but, an asymptotically and random patterns in reactor temperature

profiles, put in evidence the dynamic behaviour of reactants and products inside the reactor

due to increased internal pressure. The plot in Figure 3.7 corroborates this assumption,

showing slight higher hydrogen rates for the last loadings, in which the variation of

temperature were more pronounced.

10 wt% NaBH4 and 7 wt% NaOH | five successive loadings

00,5

11,5

22,5

33,5

44,5

55,5

0 100 200 300 400 500 600

time / sec

pres

sure

/ ba

r

1st load ; H2 yield=100%2nd load ; H2 yield=100%3rd load ; H2 yield=100%4th load ; H2 yield=100%5th load ; H2 yield=100%

Figure 3.7. Hydrogen generation in batch reactor LR (646 cm3) of five successive loadings of 10 mL each of the reactant solution: 10 wt.% NaBH4, 7 wt.% NaOH, 83 wt.% H2O, with Ni-Ru based catalyst/NaBH4: 0.4g/g.

Figure 3.8 show a typical course of hydrogen generation, presented also in terms of the

operating pressure as a function of time, for seven successive loadings of fuel solution, in

batch reactor MR (369 cm3). These results show an improvement in the performance of the

used nickel based catalyst, comparing with similar trend published by Pinto et al. [3.33],

the latter without ruthenium. In fact, the nano powdered Ni-Ru based catalyst used in the

experiments reported on this dissertation, gives higher H2 yield in a relatively short

reaction time until the ‘plateau’ is achieved.


41

0

2

4

6

8

10

12

0 100 200 300 400 500

time / s

Pre

ssu

re /

bar

1st load ; H2 yield=100% 2nd load ; H2 yield=100% 3rd load ; H2 yield=100%

4th load ; H2 yield=100% 5th load ; H2 yield=100%

Figure 3.8. Hydrogen generation in batch reactor MR (369 cm3) of seven successive loadings of the reactant solution: 10 wt.% NaBH4, 7 wt.% NaOH, 83 wt.% H2O, with Ni-Ru based catalyst/NaBH4: 0.4g/g.

As mentioned by Pinto et al. [3.33], the increasing pressure of the gas phase inside the

reactor, due to hydrogen generation, forces at the same time, the hydrogen dissolution.

This is an interesting finding since molecular hydrogen was generated but also stored in the

liquid phase.

Figure 3.9 shows the gaseous reaction products upshot after a quick exit from the

reactor, at the end of the reaction of the seven successive loading (the H2 generated in the

previous successive loadings, was released to the atmosphere).

Figure 3.9. Two images of reaction “gaseous” by-product, after seven successive loadings of reactant solution (10 wt.% NaBH4, 7 wt.% NaOH, 83 wt.% H2O). The H2 release shows the gas capability to agitate the nickel-based powdered catalyst presented at the reactor liquid phase.

As we can see, the “sparkling” by-product quickly dissipates the H2 bubbles along the

glass goblet, showing the gas capability to agitate the nickel-based bimetallic powdered


42

catalyst presented at the reactor liquid phase. This attests that a significant amount of the

H2 generated inside the reactor was trapped in the liquid reactor phase, keeping it

dissolved. Just after a slight aperture of the reactor exhaustion needle valve, due to pressure

differences, the dissolved gas (H2 plus air) is released.

3.3.6.1 Effect of agitation on hydrogen reaction rate

In addition, and with the purpose of verifying the influence of agitation on hydrogen

reaction rate during successive loadings of fuel, a TeflonTM magnetic stirrer was placed

inside the reactor prior to the first loading injection. The series of experiments performed

on this section, the Ni-Ru based powdered catalyst was reused between 226 and 233 times.

Vigorous magnetic stirring was allowed during the formation of hydrogen by injection

of the 8th loading of reactant in a series of experiments by successive refuelling, to promote

complete mass transfer (to overcome diffusion limitation). Figure 3.10, shows an eligible

increase in yield and rate of H2, lowered by a minor reaction induction time (by comparing

the results with the previous 6th an 7th loadings). This leads us to infer that mass transfer

limitations exist between the reacting solution and the reused (233 times!) catalyst.

0

1

2

3

4

5

0 1000 2000 3000 4000

Reaction time /s

Pre

ssu

re /

bar

1st load ; yield=89% 2nd load ; yield=89%3rd load ; yield=88% 4th load ; yield=88%5th load ; yield=89% 6th load ; yield=88%7th load ; yield=90% 8th load ; yield=94% (MAGNETIC STIRRING)

theoretical yields

Figure 3.10. Hydrogen generation in batch reactor LR (646 cm3) of eight successive loadings of 10 mL of the reactant solution: 10 wt.% NaBH4, 7 wt.% NaOH, 83 wt.% H2O, with Ni-Ru based catalyst/NaBH4: 0.4g/g (the 8th loading was performed with magnetic stirring), at 25 ºC.


43

3.4 REACTION BY-PRODUCT CHARACTERIZATION

The by-products of the hydrogen generation reactions of all types of experiments reported

on this chapter were analyzed by X-Ray Diffractometry. Suitable crystals – see Figure

3.11, were obtained by slow evaporation of a water solution at uncontrolled environmental

conditions.

Accordingly to publish literature, boric oxides come in white rhombic crystals or

colourless, semi-transparent vitreous granules or flates, or hygroscopic lumps or powder

(the crystals, usually forms a glass, have specific gravity’s of 1.84-2.46 and melt at 450 ºC)

[3.49].

Figure 3.11. Crystals pictures of the by-product classic hydrolysis of sodium borohydride, obtained by slow evaporation of a water solution at room temperature (≈ 25 ºC).

3.4.1 Materials and methods

The obtained crystals were found to be orthorhombic, space group Pca21, cell volume

V=364.62(4) Å3, a = 10.7252(7) Å, b = 5.2525(3) Å, c = 6.4724(4) Å (uncertainties in

parentheses). There are four molecules per unit cell, calculated density 2.237 g/cm3.

Diffraction data were collected at 293 K with a Gemini PX Ultra equipped with MoKα

radiation (λ=0.71073 Å). A total of 562 independent reflections were measured, of which

501 were observed (I>2σ(I)). The structure was solved by direct methods using SHELXS-

97 [3.50] with atomic positions and displacement parameters refined with SHELXL-97

[3.51]. The non-hydrogen atoms were refined anisotropically and the hydrogen atoms were

refined freely with isotropic displacement parameters. The refinement converged to R (all

data) = 3.93% and wR2(all data) = 9.92%.

3.4.2 Results and discussion

The crystal structure of the reaction (3.1) by-product reveals that the boron atoms are in a

triangular configuration with three oxygen atoms, with B-O bond lengths between 1.27 and


44

1.29 Å. The BO3 triangular arrangement was already found in the crystal structure of

orthorhombic [3.52] and monoclinic [3.53] metaboric acid although in the title crystal

structure the oxygen atoms are bound to just one B atom. The BO3 triangles form layers

3.238 Å apart that sandwich a layer of sodium and water molecules. For each boron atom

there are three bounded oxygen atoms and one water oxygen.

Half of the sodium atoms have their valence strength distributed between six Na-O

bonds and the other half between seven Na-O bonds (as already determined in other

sodium metaborate structures [3.54]). The water molecules are involved in a net of

interactions with the sodium atoms. Figure 3.12, below, shows two different views of the

crystal structure.

(a) (b)

Figure 3.12. Views of crystal structure of sodium metaborate dehydrate showing stacking layers formed by BO3 triangles and by Na and water molecules: (a) view along the a-axis and (b) view along the c-axis. Na in violet, O in red, B in pink and H in white.

We may conclude that the reaction (3.1) by-product, in all the types of experiments

performed by alkali hydrolysis of NaBH4 under pressure, is a sodium metaborate

dehydrated, NaBO2.2H2O, as expected. Therefore, equation (3.1) can be re-written as

follows:

NaBH4 + 4 H2O → NaBO2.2H2O + 4 H2 + heat . (3.6)

3.5 CONCLUSIONS

In this chapter the results of an extensive experimental work done with the objective of

studying various effects in hydrogen generation rate, yield and lag time, by catalytic alkali

hydrolysis of sodium borohydride, are reported. Briefly, the main conclusions are:


45

- increasing the temperature of the reaction medium, increases the rate of H2 generation;

- the reaction rate increases with increasing the hydride concentration up to 20 wt% of

NaBH4. Above this value, a slight decrease on both H2 and yield was found;

- the reaction rate is greatly enhanced by the increase of the NaOH concentration, ensuring

good efficiency of hydrogen generation;

- an average yield of ≈ 90% was found in H2 generation; this is substantially lower than the

maximum predicted hydrogen pressure, assuming 100% conversion and applying the ideal

gas law. Pinto et al. [3.33] suggests that this fact is related with solubility effects in the

liquid phase that remains inside the reactor;

- the reactor bottom shape affects the course of hydrogen generation. It was found that the

conical bottom geometry leads to an increase on reaction rate and yield, and slightest in H2

time lag;

- the Ni-Ru based catalyst has a good performance in refueling experiments. In fact, it was

found that the catalyst tends to perform better, after the first loading of reactant (the

amount of catalyst remains the same from the first to the last loading);

- a vigorous stirring of the solution inside the batch reactor, during the experiments with

successive fuel loadings, reveals minor reaction induction time and a slight higher yield in

hydrogen generation. This indicates possible mass transfer limitations during the course of

reaction between the fuel and the active sites of the catalyst.

The x-ray diffractions analysis revealed a sodium metaborate dehydrated,

NaBO2.2H2O, as the by-product of all experiments performed in the several subsections of

this chapter.

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[3.44] Ö. Metin, S. Özkar, Hydrogen generation from the hydrolysis of sodium borohydride by using water dispersible, hydrogenphosphate-stabilized nickel(0) nanoclusters as catalyst, Int. J. Hydrogen Energy 32 (2007) 1707-1715.

[3.45] M. Mitov, R. Rashkov, N. Atanassov, A. Zielonka, Effects of nickel foam dimensions on catalytic activity of supported Co-Mn-B nanocomposites for hydrogen generation from stabilized borohydride solutions, J. Mater. Sci. (42) 2007 3367-3372.

[3.46] K.W. Cho, H.S. Kwon, Effects of electrodeposited Co and Co-P catalysts on the hydrogen generation properties from hydrolysis of alkaline sodium borohydride solution, Cat. Today 120 (2007) 298-304.

[3.47] K. Eom, K.W. Cho, H.S. Kwon, Effects of electroless deposition conditions on microstructures of cobalt-phosphorous catalysts and their hydrogen generation properties in alkaline sdium borohydride solution, J. Power Sources 180 (2008) 484-490.

[3.48] J.H. Park, P. Shakkthivel, H.J. Kim, M.K. Han, J.H. Jang, Y.R. Kim, H.S. Kim, Y.G. Shul, Investigation of metal alloy catalyst for hydrogen release from sodium borohydride for polymer electrolyte membrane fuel cell application, Int. J. Hydrogen Energy 33 (2008) 1845-1852.

[3.49] F. A. Cotton, G. Wilkinson, in “Advanced Inorganic Chemistry”, John Wiley & Sons, 5th Edition, Chapter Six, p.162.

[3.50] G.M. Sheldrick, SHELXS-97, Program for the solution of crystal structures; University of Göttingen: Germany 1997.

[3.51] G.M. Sheldrick, SHELXL-97, Program for the refinement of crystal structures; University of Göttingen: Germany 1997.

[3.52] H. Tazaki, The structure of orthorhombic metaboric acid, HBO2 (α), J. Sci. Hiroshima Univ. A10, 37 (1940) 55-61.

[3.53] W.H. Zachariasen, The crystal structure of monoclinic metaboric acid, Acta Cryst. 16 (1963) 385-389.

[3.54] M. Marezio, H.A. Plettinger, W.H. Zachariasen, The bond lengths in the sodium metaborate structure, Acta Cryst. 16 (1963) 594-595.

49

CHAPTER 4

Alkali free hydrolysis Results for sodium borohydride kinetic experiments

This chapter shows the results obtained on alkali free hydrolysis of sodium borohydride in the presence of a reused nickel based bimetallic catalyst. A discussion on the stoichiometry of hydrolysis, in terms of the number of added water molecules to solid NaBH4, is carried out, based on the results of hydrogen generation rate and yields. Emphasis is given to reactor bottom design effects on the hydrogen production. The reaction by-products are analysed by XRD.

4.1 INTRODUCTION

The main inspiration behind developing this experimental item was to be capable of

producing pure molecular hydrogen with very high gravimetric and volumetric densities.

The gravimetric and volumetric energy densities attainable from hydrolysis of

chemical hydrides depend in large measure on the amount of water required for the

process. It is well known that hydrolysis of sodium borohydride produces hydrated

metaborate by-products in which the degree of hydration of the metaborate is closely

joined to the amount of water present in the reaction [4.1].

In the previous chapter, attention was focused on studying the catalytic stabilized

hydrolysis of sodium borohydride in the presence of water excess, namely alkali

hydrolysis of sodium borohydride, for hydrogen generation (this reaction generates basic

species that are thermodynamically and kinetically metastable, causing the reaction to

cease at low yields of hydrogen even though the reaction is strongly favoured

thermodynamically [4.2]). The results revealed that when working at high pressures, and in

the presence of a Ni-Ru based catalyst, even with water in excess, it is possible to produce

CHAPTER 4 Alkali free hydrolysis

50

compressed H2 at ≈ 99% yields. The alkali hydrolysis by-products were NaBO2.2H2O

under a variety of reaction conditions.

The objective of the study presented in this chapter, is practically the same - produce

hydrogen at high pressures, but with very high gravimetric hydrogen density. Then, instead

of dealing with an excess of water, a solid fuel plus a stoichiometric amount of pure water

was used, in the absence of an alkali inhibitor. In this way, it is suitable to call this research

topic alkali free hydrolysis of sodium borohydride for hydrogen generation. The latter will

occur only when the measured amount of Ni-Ru based catalyst and the solid hydride

closely tie to a fixed stoichiometric amount of pure water. Since this research topic does

not deal with NaOH, our study does not involve a strongly caustic solution which is

excellent from an environmental point of view.

As stated in Chapter 3, NaBH4 reacts with water to generate molecular hydrogen

according to the hydrolysis reaction shown in Eq.(4.1):


Ideal hydrolysis is attained for the excess hydration factor (x), 0=x [4.3], but in

practice an excess of water is required to account for the fact that the solid by-product

(NaBO2.xH2O) can exist in varying degrees of hydration [4.1].

Experimental studies showed that if the excess hydration factor (x) is less that 2 (x<2),

unreacted hydride remains; but if x>2 there is enough water for complete reaction and

hydration of the borate.

Marreo-Alfonso et al. [4.1] argued that a hydration factor less than x=0.84 is necessary

to meet the FreedomCar DOE specifications related to weight and volume hydrogen

efficiency [4.4]. Also, sodium metaborate dehydrate, NaBO2.2H2O, appears to be the

favoured product at temperature up to 110 ºC; and transforms to anhydrous metaborate after

being dried at 350 ºC, while NaBO2.4H2O becomes anhydrous above 400 ºC.

Based on these results, an excess of water seems to be always necessary to react most

of the NaBH4 used in this kind of reaction systems.

As already mentioned, this chapter focuses on the reduction of the factor x, which may

be called a “solid phase pathway”. This approach presumably minimizes the water/hydride

ratio in the reactor at any instant. This suggests that it may be possible to minimize the

excess hydration factor through a pathway that leads toward by-products with lower

hydration state. In fact, one of the major challenges for the implementation of hydride-


51

based storage technologies is to minimize the amount of water for 100% liberation of

hydrogen.

Kojima et al. [4.5] declare that with a Pt-LiCoO2 catalyst and high H2 pressure above

0.6 MPa, a nearly theoretically H2 yield using x = 0 in (4.1) was produced. There is no

doubt that if reaction (4.1) occurs in the presence of steam instead of liquid water, the solid

reaction products should contain less water of hydration, which in the long term will

facilitate product recycling and regeneration of the hydride.

It results from the statement in the last paragraph that it is valuable to study the

reaction depicted in equation (4.1) at high pressures and with 2<x<4. Even if impossible to

perform the hydrolysis reaction by feeding water in vapour phase, it is always important to

work with values of x as low as possible. Maybe working at high pressures and optimizing

the reactor design, it may be possible to achieve high H2 yields at fast rates. Nevertheless

to achieve these goals, it is absolutely necessary to fully understand the thermodynamics,

kinetics and chemistry of the hydrolysis reaction.


The alkali free catalytic hydrolysis of sodium borohydride for hydrogen generation under

pressure comprises a series of experiments done in the three batch reactors LR, MR and

SM, with internal free volumes of 646 cm3, 369 cm3 and 229 cm3, respectively.

Sodium borohydride powder (≥ 96% purity) was used in all the experiments in the

solid state, and in a constant amount of ≈1.2 g. This value was carefully chosen for

comparing the results with those obtained in the alkali catalytic hydrolysis of 10 mL of a

reactant aqueous solution 10 wt% NaBH4 plus 7 wt% NaOH, by weight (the amount of

NaBH4 in this solution is approximately 1.2 g).

As already referred, the reactant blend was produced without adding NaOH aqueous

solution to solid NaBH4. Hence, a proper quantity of powder catalyst and solid NaBH4,

both measured in an analytical balance, are mixed together in the solid state, forming a

solid powder mixture. Next, this mixture is stored at the bottom of the reactor and the

reaction vessel is closed. Figure 4.1 shows three photographs that illustrate the described

experimental procedure.

As stated in Chapter 2, after sealing perfectly the reactor, an adequate amount of H2O

was rapidly injected into the reactor by means of a syringe with a very high needle length

(150 mm) to ensure that the water is delivered very close to the powder moisture (NaBH4

plus catalyst).


52

Figure 4.1. Pictures showing the preparation of the reactant blend (catalyst plus NaBH4, both solids) to use in alkali free hydrolysis experiments. From left to right: Ni-Ru based catalyst, anhydrous sodium borohydride and the bottom of MR reactor with the mixture (stored in bottom conical shape).

Four different studies were performed in this topic of alkali free hydrolysis of sodium

borohydride with the objective of studying the effects on hydrogen generation rates and

yields, by changing:

i) the stoichiometric amount of added water, H2O/NaBH4 (mol/mol), from ratios 2 to 8

mol;

ii) the quantity of catalyst, catalyst/NaBH4 (g/g), for ratios of 0.2 and 0.4 g;

iii) the system pressure, performing experiments in the three designed reactors (LR,

MR and SM), which have different internal free volumes;

iv) reactor bottom shape. Here, the NaBH4 hydrolysis performance is compared in LR

reactor (flat bottom) and in MR reactor (conical bottom), and

v) the system temperature.

All the experimental work reported on this chapter was done without magnetic stirring.


Hydrogen yield = n(H2)exp / n(H2)theoretical , (4.2)


theoretical amount of generated H2 assuming 100% conversion of NaBH4 by applying the

ideal gas law to the final volume of gas inside the reactor.


As already referred, hydrogen was generated from catalysed hydrolysis of powder NaBH4

with specific stoichiometric amounts of deionised water. The effects of ratios H2O/NaBH4

(mol/mol) and Ni-Ru based catalyst/NaBH4 (g/g), system pressure, reactor bottom


53

geometry and reaction temperature on the hydrogen generation yield, rate and induction

time were studied, and the results found are next presented and discussed separately.

Kojima et al. [4.5] published experimental work on compressed H2 generation using

NaBH4 at pressures up to 5.6 MPa. For this reason, the present results, also on compressed

H2 generation, will be closely compared with those of Kojima et al. [4.5]. Excluding their

work and those of Pinto et al. [4.6], the majority of the papers published on H2 generation

from hydrolysis of NaBH4 are at ambient pressure; therefore, they will be hardly ever

mentioned on this section of the dissertation.

4.3.1 Effect of H2O/NaBH4 (mol/mol) on hydrogen generation yield

Figure 4.2 shows the course of hydrogen generation in terms of H2 pressure as a function

of time, for the batch reactor LR (646 cm3), with a flat bottom shape, by changing the

amount of H2O/NaBH4 (mol/mol) ratio from 2 to 8 mol. A proportion of Ni-Ru based

catalyst/NaBH4: 0.4 g/g was used in all the experiments depicted in that figure and the

catalyst for the experiment H2O/NaBH4: 2 mol/mol was 27 times reused, and for the

others, between 109 and 111 times reused.

0

1

2

3

4

5

6

0 2000 4000 6000 8000 10000

Time / sec

Pre

ssur

e H 2

/ b

ar

H2O/NaBH4: 2 mol/mo | H2 yield=47%l H2O/NaBH4: 4 mol/mol | H2 yield=78%

H2O/NaBH4: 6 mol/mol | H2 yield=92% H2O/NaBH4: 8 mol/mol | H2 yield=98%

Theoretical Yield

0

0,5

1

0 250 500

Figure 4.2. Hydrogen generation in the flat bottom batch reactor LR (646 cm3) for experiments with Ni-Ru based catalyst/NaBH4: 0.4 g/g and H2O/NaBH4: 2-8 mol/mol.

The results in the Fig.4.2 show the effect of changing the molar ratio H2O/NaBH4 on

H2 yield and rates. In fact, increasing the number of water molecules, the H2 generation is

achieved not only at higher yields but also a faster rate (for example, for H2O/NaBH4: 8


54

mol/mol, the reaction attained completion (time ≈ 83 min) with the highest dtdP / slope in

the linear zone, 1.47E-3 bar/s or 0.11 L(H2)min-1gcat-1).

For the experiment with H2O/NaBH4: 2 mol/mol illustrated in Fig.4.2, a hydrogen

yield of about 47 % was reached in 125 min, at 0.22 MPa ( dtdP / slope in the linear zone,

3.49E-4 bar/s or 0.026 L(H2)min-1gcat-1). Kojima et al. [4.4], reported an H2 yield of about

80% for the same molar relation and Pt-LiCoO2/NaBH4: 0.5 g/g, after 400 s, at 0.68 MPa

and 296 K. They also reported, for the same experiment, but at atmospheric pressure (0.10

MPa), only 37%, with the plateau reached on 330 s. In fact, the substantial lower yield of

47 %, obtained in this work, can be explained by both the catalyst nature – Kojima et al.

[4.5] used a Pt-LiCoO2 catalyst, and a higher operating pressure (0.68 MPa).

In Figure 4.2 an enlargement of the left bottom corner of the plot is also represented.

The aim is to check the reaction lag time of the experiments. As can be seen, the H2

generation starts immediately after the injection of water, even at slow rate.

To solve the incompleteness of reaction (1) when H2O/NaBH4: 2 mol/mol (note that a

theoretical final pressure of 0.46 MPa was expected), the reaction vessel was open and a

second loading of pure water was injected with the same H2O/NaBH4 (mol/mol) ratio

amount. This new addition of fresh water reacted with the left unreacted NaBH4 present

inside the batch reactor. Figure 4.3 shows the result obtained after injecting a second

H2O/NaBH4: 2 mol/mol of distilled water.

As expected, the amount of hydrogen generated by the second injection of pure water

was approximately the same registered in the first loading of water, and was produced at

similar rates (in the linear zone, dtdP / slopes are quite similar, showed by the parallel

straight lines for both water injections).

In Figure 4.4, the influence of added water, H2O/NaBH4 (mol/mol), on the hydrogen

yield is studied and the results presented on this dissertation (at 293-295 K with a Ni-Ru

based/NaBH4: 0.4 g/g) are compared with those of Kojima et al. [4.5] (at 296 K with Pt-

LiCoO2/NaBH4: 0.5 g/g). It can be observed that the H2 yield increases with increasing

H2O/NaBH4 ratio. For example, for the results presented on this dissertation, increasing the

molar ratio H2O/NaBH4 from 2 to 6 mol/mol, the corresponding H2 yield goes from 47 to

92% (these results arise at low pressure range, between 0.22 and 0.45 MPa). Therefore, for

H2O/NaBH4 equal to 8 mol/mol, completion is achieved.


55

0

1

2

3

4

5

0 10000 20000 30000 40000 50000 60000

Time / sec

Pre

ssur

e H 2

/ b

ar

1st loading of pure water 2nd loading of pure water

Theoretical Yield

Ni-based/NaBH4: 0.4 g/gH2O/NaBH4: 2 mol/mol

Pressure: 0.22 MPa

Figure 4.3. Hydrogen generation in the batch reactor LR (646 cm3) with Ni-Ru based/NaBH4: 0.4 g/g (27 and 28 times reused) and H2O/NaBH4: 2 mol/mol, after two successive loadings of pure water (of the same molar quantity).

0

20

40

60

80

100

0 2 4 6 8H2O/NaBH4 / mol/mol

Hyd

rog

en y

ield

/ %A

This work | Low pressure (0.2-0.4 MPa) and Ru-Ni based/NaBH4: 0.4 g/g

Kojima et al. (2004) | Atmospheric pressure (0.10 MPa) and Pt-LiCoO2/NaBH4: 0.5 g/g

Kojima et al. (2004) | High pressure (5.0-5.6 MPa) and Pt-LiCoO2/NaBH4: 0.5 g/g

Figure 4.4. Hydrogen yield as a function of added water, H2O/NaBH4, in batch reactor LG (646 cm3).

Regarding again the previous figure, Kojima et al. [4.5] reported a strikingly sharp

increase in H2 yield as H2O/NaBH4 departs from 1 to 2, with values of 35% up to 87%. For

the case of H2O/NaBH4 equal to 6 mol/mol, a H2 yield of 95% was obtained. Note that all

those values were obtained at very high pressure (5.0-5.6 MPa). For atmospheric, and for


56

results of the present work - obtained at moderate pressures, high values of H2 yield are

achieved (≥ 80%) if H2O/NaBH4 ≥ 4 mol/mol.

4.3.2 Effect of catalyst/NaBH4 (g/g) on hydrogen generation rate

Figure 4.5 shows the influence of the amount of catalyst used in hydrogen rate and lag time

of an alkali free hydrolysis of sodium borohydride experiment, performed in batch reactor

LR (646 cm3).

0

1

2

3

4

5

6

0 5000 10000 15000 20000 25000 30000

Time / sec

Pre

ssu

re H 2

/ b

ar

Ru-Ni based/NaBH4: 0.2 g/g | H2 yield=78% Ru-Ni based/NaBH4: 0.4 g/g | H2 yield=78%

Theoretical Yield

Figure 4.5. Influence of the amount of catalyst/NaBH4 (g/g) on hydrogen generation.

Actually the amount of catalyst is another variable that influences the rate and the lag

time. As can be seen in the plot of Fig.4.5, increasing the amount of the catalyst

(catalyst/NaBH4: 0.2 g/g → 0.4 g/g) increases the dtdP / slope in the linear zone (2.5E-4

bar/s or 0.037 L(H2)min-1gcat-1→ 5.5E-4 bar/s or 0.041 L(H2)min-1gcat-1) and decreases

the time to reach the ‘plateau’, but, as expected, the pressure of produced hydrogen was

similar (≈ 0.36 MPa and H2 yield of 78 %) and independent of the temperature.

4.3.3 Effect of pressure on hydrogen generation yield and rate

Figure 4.6 shows the rate of hydrogen generation in terms of H2 pressure as a function of

time, for the three batch reactor LR, MR and SR, at the stoichiometric amount of 4 moles

of distilled water, using a proportion Ni-Ru based/NaBH4: 0.4 g/g. The values of H2 yields

achieved are also reported.


57

The plot in Figure 4.7 shows the H2 generation yield versus pressure, for the three

studied stoichiometric amounts of water, using Ni-Ru based catalyst (Ni-Ru based/NaBH4:

0.4 g/g), and also the results obtained by Kojima et al. [4.5], for comparison.

0

2

4

6

8

10

12

14

0 6000 12000 18000

Time / sec

Pre

ssu

re H 2

/ b

ar

LR; H2 Yield: 78% MR; H2 Yield: 86% SR; H2 Yield: 98%

Figure 4.6. Hydrogen generation in the studied three batch reactors - LR (646 cm3), MR (369 cm3) and SR (229 cm3), with Ni-Ru based catalyst/NaBH4: 0.4 g/g (111, 150 and 151 times reused, respectively for LR, MR and SR) and H2O/NaBH4: 4 mol/mol.

0

20

40

60

80

100

0 1 10 100

Pressure / MPa

Hyd

roge

n yi

eld

/ %

This work

Kojima et al. (2004)

Ni-based/NaBH4: 0.4 g/g

H2O/NaBH4: 4 mol/mol

Pt-LiCoO2/NaBH4: 0.5 g/g


Figure 4.7. Influence of pressure on H2 yield (Experiments done in three batch reactors at the temperature range of 289-295 K, with catalyst reused ≈150 times).

As can be seen from the plots of Fig.4.6-4.7, the effects of pressure on H2 yields and

rates are remarkable. In truth, we may conclude that increasing the pressure, significantly

increases H2 rates and H2 yields.


58

For the operating conditions presented on this dissertation, and for the special case of

H2O/NaBH4: 4 mol/mol, performed at the moderate pressures between 0.36-1.26 MPa,

values for H2 yields up to 78-98% were obtained (see Fig.4.7).

Kojima et al. [4.5] reported values of 80-93% for H2 yields under much higher

pressure levels (0.7-25 MPa), with H2O/NaBH4: 2 mol/mol (see Fig.4.7).

It is worth to notice that the results presented in Fig.4.6 were performed using Ni-Ru

based powdered catalyst reused ≈150 times, in batch reactors with flat and conical

bottoms. As can be easily checked, the catalyst shows high activity under moderate

pressure resulting in a significant increase in the amount of hydrogen generated. Kojima et

al. [4.5] conclude the same trend (at H2O/NaBH4: 2 mol/mol) with a noble catalyst Pt-

LiCoO2/NaBH4: 0.5 g/g at higher H2 pressure, up to 25 MPa.

4.3.4 Effect of reactor bottom shape on hydrogen generation

Looking back to the results plotted in Fig.4.6, presented in the previous section, it seems

very important to study the effect of reactor bottom shape on H2 generation rate and yield.

The plot in Figure 4.8 show the H2 generation trend for H2O/NaBH4: 2 mol/mol and

Ru-Ni based/NaBH4: 0.4 g/g., in batch reactors LR (646 cm3) and MR (369 cm3), with flat

and conical bottom shapes, respectively.

0

2

4

6

8

0 2000 4000 6000 8000 10000 12000

Time / sec

Pre

ssu

re H 2

/ b

ar

LR - FLAT bottom; H2 Yield: 78% MR - CONICAL bottom; H2 Yield: 86%

0

2

4

6

8

0 20 40 60 80

Figure 4.8. Influence of reactor bottom shape on H2 yield, rate and lag time (experiments performed in batch reactors LR (646 cm3) and MR (369 cm3), with flat and conical bottom shapes, respectively; at temperature range of 289-295 K, with catalyst reused ≈150 times).

As can be observed from the plot of Fig.4.8, the influence of reactor bottom shape on

H2 generation is notorious: the conical geometry greatly enhances the rate and yield of H2


59

generation. For the batch reactor MR, with conical bottom, a value of dtdP / slope in the

linear zone of 5.7E-1 bar/s (24.2 L(H2)min-1gcat-1) was found. In fact, a difference of 103

separates this value from that obtained in the reactor with a flat bottom shape (5.8E-4 bar/s

or 0.043 L(H2)min-1gcat-1), for the same H2 reaction.

An enlarged plot on the left side of Fig.4.8 put in evidence the sharply increase of H2

pressure by time (confirmed by the high value in dtdP / slope mentioned in the previous

paragraph), after a period of ≈36 seconds in lag time (the lag time is defined as the length

of time required to observe an increase in the H2 pressure). In reality, the reactor bottom

shape influences also the H2 lag time! Observing Fig.4.8 we may conclude that the conical

reactor bottom geometry significantly decreases the lag time. A plausible explanation is

that the reactor conical bottom shape enhances the contact between the catalyst and NaBH4

powders, and the injected distilled water. The photographs in Figure 4.9, below, outline

this aspect: the MR reactor (left side) when opened after the experiment which results are

plotted in Fig.4.8, shows absence of liquid and looks empty. The photograph on the right

side of Fig.4.9 shows the reaction by-product plus catalyst, in a form of a swelled

aggregated porous solid, shaped by MR reactor conical bottom, which adhered to the

T.Bottom k thermocouple, the latter fixed on the top of the reactor lid.

Figure 4.9. Photographs of MR reactor (with conical bottom shape) after an alkali free hydrolysis of NaBH4 experiment. In the right, a view of the solid mixture of the by-product plus the catalyst.

The sharp increase of H2 pressure with time shown in Fig. 4.8, which occurred in the

MR batch reactor with a conical bottom, is in some way a consequence of also a sharp

increase in the value of temperature of the hydrolyzed NaBH4. Figure 4.10 illustrates the

variation of reaction temperature occurred during the hydrogen generation for the two

experiments plotted in Fig 4.8.


60

250

300

350

400

450

0 200 400 600 800 1000

Time / sec

Tem

per

atur

e /

K

H2 presure in LR: 0.36 MPa

H2 presure in MR: 0.69 MPa


Ru-Ni based/NaBH4: 0.4 g/g

Figure 4.10. Temperature of the hydrolyzed NaBH4 as a function of time by different pressures.

Indeed, in the MR batch reactor, the occurrence of a ‘thermal runaway’ which was

responsible for the increase of about 150 ºC in the reaction temperature, happened in a

short time interval, less than one minute, and consequently, accelerated harshly the

hydrolysis reaction.

The temperature in the batch reactor (with a conical bottom), at 0.69 MPa, shows the

maximum value of 428 K after the induction period, and decreases quickly with time. It

may be conclude that when the temperature of hydrolyzed hydride increases the induction

period decreased. A similar trend was found by Kojima et al. [4.5].

4.3.5 Effect of temperature on hydrogen generation rate

The influence of temperature on the velocity of hydrogen generation is put in evidence in

Figure 4.11.

As can be seen in the plot of Fig.4.11, the reaction rate is very sensitive to temperature.

As expected, the rate rises with the increase in temperature and the hydrogen pressure

demonstrates a near linear variation with reaction time. The influence of temperature is

clearly shown by the increasing slope values on the linear region of the plots, for

increasing values of the reaction temperature (from 15 to 55 ºC, the rate increases from

5.4E-4 to 4.4E-1 bar/s, or 0.021 L(H2)min-1gcat-1 to 14.4 L(H2)min-1gcat-1). Therefore, it

seems that the reaction rate remains practically constant when, during the hydrolysis

process, the NaBH4 concentration decreases due to hydride and water consumption,

demonstrating a zero-order reaction. A more detailed study of the effects of reactants

concentration, operating pressure and temperature is necessary; in order to develop a


61

kinetic model that accurately reflects the reaction rate as a function of temperature and

pressure.

0

2

4

6

8

10

12

0 12500 25000

time / s

Pre

ssur

e /

ba

r

15 ºC | yield=87%

25 ºC | yield=89%

35 ºC | yield=83%

45 ºC | yield=75%

55 ºC | yield=89%

Figure 4.11. Influence of temperature on hydrogen generation rate and yield.

4.3.6 Gravimetric and volumetric densities

It is here worth to remember that the main purpose of the alkali free hydrolysis of NaBH4

experiments reported in this dissertation is to produce pure hydrogen gas with very high

gravimetric and volumetric densities. And the ability to produce anhydrous sodium borate

is the key to increase the overall storage density of systems based on sodium borohydride

as the storage media [4.1, 4.5].

Kojima et al. [4.5] reported that the structure of the by-product produced from the

reaction of NaBH4 and water using Pt-LiCoO2, is NaBO2.2H2O at atmospheric pressure,

and anhydrous NaBO2 at high pressure. They also mentioned that the temperature of the

hydrolyzed NaBH4 is higher in the catalyzed reaction at 0.10 MPa than at 0.68 MPa. A

possible explanation can be related with the energy required for the addition of two water

molecules in the heat of formation of NaBO2.2H2O. In some way, similar behaviour was

found in the present work (see Figs. 4.8 and 4.10, above). In fact, if we look through the

plot of Fig.4.6, the H2 yield increases from 86% to 98% in the MR (369 cm3) and SR (229

cm3) batch reactors, both with conical bottom shape. Why? An inspection of the

temperature profile inside of SR reactor shows a lower reaction temperature; maybe

because the H2 is not consumed to hydrate sodium borate by-product. It is therefore


62

important to investigate the type of by-products formed; and that will be done in Section

4.4.

The US Department Of Energy (DOE) has published FreedomCAR requirements for

automotive hydrogen storage systems (probably the most stringent targets yet articulated

for hydrogen storage systems). Two of the principal technical targets, based on system

mass and volume, are the gravimetric hydrogen density and the volumetric hydrogen

density. Table 4.1 shows these FreedomCAR two specifications [4.4].

Table 4.1 - The FreedomCAR targets publish by US Department Of Energy (DOE).

Year 2007 2010 2015

Gravimetric H2 density / wt.% 4.5 6.0 9.0

Volumetric H2 density/ kgH2/m3 36 45 81

(Note that these specifications are based on system mass and volume, i.e., not only the storage

material itself but also the reactors, tanks, valves, and all auxiliary equipment, which usually are

call hardware.)

Table 4.2 below, presents the predicted values for gravimetric hydrogen density and

volumetric hydrogen density, based only on the storage material itself, for the systems

studied in this chapter: H2O/NaBH4: 4 mol/mol and Ru-Ni based catalyst/NaBH4: 0.2-0.4

g/g. Hence, the influence of Ni-Ru based catalyst/NaBH4 on gravimetric hydrogen density

and on hydrogen yield is also checked.

The density of the material (H2O/NaBH4: 4 mol/mol; Ni-Ru based/NaBH4: 0.2-0.4

g/g) was calculated by adding the densities of NaBH4 (1.07 g/cm3), water (1.00 g/cm3) and

Ni-Ru based catalyst (3.17 g/cm3).

It was found that our system of compressed hydrogen generation at moderate pressure

of up to 1.26 MPa is currently capable of meeting 6.3 wt% and 70 kg H2/m3, respectively,

for gravimetric hydrogen density and volumetric hydrogen density, based only on the

storage material itself (and not on the storage system as a hole or including hardware).

Hence, it is not correct to compare the values obtained with those in Table 4.1, the 2010

FreedomCAR targets published by the US DOE. Can we ask: Is NaBH4 a suitable

hydrogen storage material for portable and/or niche application? It is difficult, indeed, to

answer this question. To the best of the authors’ knowledge there is no paper or document

clearly specifying the targets for portable applications.


63

Table 4.2 - Influence of Ni-Ru based catalyst/NaBH4 on gravimetric hydrogen density and hydrogen yield for H2O/NaBH4: 4 mol/mol. Prediction of the volumetric hydrogen density*.

Ni-based/NaBH4 / g/g Gravimetric H2 density / wt.% H2 yield / % Volumetric H2 density* / kgH2/m3

Pressure / MPa

0.2 5.3 78 57 0.36

0.4 5.0 78 56 0.36

0.4 5.3 86 59 0.690.4 6.3 98 70 1.26

* It was assumed that the average value of the density of our materials, by addition relationship of densities of NaBH4 (1.07 g/cm3), H2O (1.00 g/cm3) and Ni-based catalyst (3.17 g/cm3) is:

1.07 g/cm3, for H2O/NaBH4: 4 mol/mol, Ni-based catalyst/NaBH4: 0.2 g/g; and

1.11 g/cm3, for H2O/NaBH4: 4 mol/mol, Ni-based catalyst/NaBH4: 0.4 g/g

Even for the purpose of a didactic demonstration, the MicroBoro Bus plus the H2

generation system, presented here, delivers pure H2 in high rates, during a good fraction of

time/autonomy. Hence, it seems that NaBH4 has potential for portable applications, or at

least niche applications. In a recent paper, the values 3-7 wt% for gravimetric hydrogen

storage capacities for NaBH4 systems are mentioned [4.7].

It is important referring that sustainable method to recycle the borate by-products back

to NaBH4 would be a very important obstacle to overcome for the adoption of NaBH4 as a

hydrogen carrier of choice for portable H2 fuel cell applications.


The by-products of the alkali free hydrolysis experiments for hydrogen generation,

reported on this chapter, were analyzed by X-Ray Diffractometry. As already referred

suitable crystals, from the reactions performed under and above 1 MPa pressure, were

obtained by slow evaporation of a water solution at uncontrolled room temperatures. The

crystal structures were alike with respect to water content.

4.4.1 Materials and methods

For the crystals coming through MR reactor experiments (at 0.69 MPa), XRD analysis

revealed that they were monoclinic, space group C2/c, cell volume V=1478.8(2) Å3. Unit

cell parameters a = 11.8899(9) Å, b = 10.6394(8) Å, c = 12.1989(9) Å, β = 106.609(8)º.

Diffraction data were collected at 293 K with a Gemini PX Ultra equipped with MoKα

radiation (λ=0.71073 Å). The structure was solved by direct methods using SHELXS-97

[4.8] with atomic positions and displacement parameters refined with SHELXL-97 [4.9].

The non-hydrogen atoms were refined anisotropically and the hydrogen atoms were


64

refined freely with isotropic displacement parameters. The refinement converged to R (all

data) = 5.30% and wR2 (all data) = 11.87%.

4.4.2 Results and discussion

The crystal structure reveals the B atoms in triangular and tetragonal configuration with O

atoms. The Na atoms are coordinated by six water molecules. The molecular formula is

Na2B2O5.4H20. Less two water molecules were similarly find in the crystal structure in the

SR batch reactor experiments (at 1.26 MPa).

Figure 4.12, below, shows the two crystal structures discovered for the sodium

metaborate hydrates.

It may be concluded that the use of Ru-Ni based catalyst, that shows high activity in

alkali free hydrolysis under high pressure, revelled not only high yields of hydrogen

generation, as reported in the above sections, but also, for pressure > 1 MPa, a change in

the structure of the by-product from Na2B2O5.4H20 to two less water content. This is an

important finding due to gravimetric hydrogen density and borates recycle.

(a) (b)

Figure 4.12. View along the c-axis of the crystal structure of sodium metaborate hydrate: (a) 4H2O, in MR at 0.69 MPa and (b) 2H2O, in SR at 1.26 MPa. The element color code, from black to light grey, is B, O, Na and H.

4.5 CONCLUSIONS

The fundamental understanding of the alkali free hydrolysis of sodium borohydride

obtained in this chapter suggests that it is possible to develop a hydrogen reactor/delivery


65

system that produces pure hydrogen in ≈100% yield, requires a suitable catalyst, does not

involve strongly caustic solutions, and uses a minimum/stoichiometric amount of water.

Chemical hydrides have the potential to meet certain H2 storage targets only if the

water consumption is minimized. A key thermophysical requirement is to minimize the

“excess hydration factor”, as mentioned in this chapter. An alkali free hydrolysis under

pressure may help to achieve this. Experimentally, we have shown for a system with

H2O/NaBH4: 4 mol/mol and Ru-Ni based/NaBH4: 0.4 g/g, performed in a batch reactor

with conical bottom shape (229 cm3), that the H2 produced reaches almost 100%

conversion of NaBH4, delivered in fast rate and without significant lag time, when the

operating pressure is > 1 MPa.

4.6 REFERENCES

[4.1] Marrero-Alfonso E, Gray JR, Davis TA, Matthews MA. Minimizing water utilization in hydrolysis of sodium borohydride: the role of sodium metaborate hydrates. Int. J. Hydrogen Energy 32 (2007) 4723-4730.

[4.2] C.M. Kaufman, S. Buddhadev, Hydrogen generation by hydrolysis of sodium tetrahydroborate: effects of acids and transition metals and their salts, J. Chem. Soc. Dalton Trans. (1985) 307-313.


[4.4] U.S. Department of Energy Hydrogen Program. Available from: http://www.hydrogen.energy.gov/.

[4.5] Y. Kojima, Y. Kawai, H. Nakanishi, S. Matsumoto, Compressed hydrogen generation using chemical hydride, J. Power Sources 135 (2004) 36-41.


[4.7] U.B. Demirci, O. Akdim, P. Miele, Ten-year efforts and a no-go recommendation for sodium borohydride for on-board automotive hydrogen storage, Int. J. Hydrogen Energy

34 (2009) 2638-2645.

[4.8] G.M. Sheldrick, SHELXS-97, Program for the solution of crystal structures; University of Göttingen: Germany 1997.

[4.9] G.M. Sheldrick, SHELXL-97, Program for the refinement of crystal structures; University of Göttingen: Germany 1997.


66

67

CHAPTER 5

less Polar Organic Polymeric Solutions hydrolysis Results for sodium borohydride kinetic experiments

This chapter focuses on the results of small additions of an organic polymer or surfactant to the stabilized catalytic hydrolysis of sodium borohydride for simultaneous hydrogen generation and storage in the liquid phase. The effects of pressure and reactor bottom shape on the hydrogen generation rate are investigated. Particular importance is given to H2 solubility’s effect in the liquid phase by increasing the operating pressure. XRD analysis for the reactions by-product is also presented.

5.1 INTRODUCTION

Hydrogen is being focused as the energy source that will replace the current fossil fuel in

the 21st century. Pure hydrogen is especially interesting for fuel cell technologies. In fact,

fuel cells are alternative power sources for providing clean energy for transportation and

personal electronic applications where low system weight and portability are important.

But produced hydrogen needs to be stored effectively and safely, with high

gravimetric/volumetric storage capacities. This represents the biggest scientific challenge

to overcome before the implementation of an economy based on this energy carrier

becomes possible.

Currently, there is a great interest in the use of solid state materials for the storage of

hydrogen. Hydrides of light elements may represent the only viable method of achieving a

high weight percent and high volume density of stored hydrogen; in particular, sodium

borohydride (NaBH4) has such features [5.1].

In November 2007 the U.S. Department of Energy (DOE) recommended a no-go for

NaBH4 for on-board automotive hydrogen storage [5.2] and, we have to admit, that

decision is completely justified. In fact, the 2015 targets for gravimetric (9.0 wt%) and

CHAPTER 5 less Polar Organic Polymeric Solutions (lPOPS) hydrolysis

68

volumetric (81 kg/m3) hydrogen storage densities (calculated on 1 kg system-basis),

established by the FreedomCAR US DOE [5.3] program (which probably is the most

stringent yet articulate for hydrogen storage systems) revealed some penalties. The latter

result from the fact that it is obligatory to take into account, on specific H2 energy

calculations, the weight and volume not only of the materials but also of the hardware (i.e.,

the reactors, tanks, valves and all auxiliary equipment) to achieve those targets. Although

the intentions of FreedomCAR specifications were to stimulate breakthroughs, both in

material storage capacities and in efficient system design, the fact is that system

requirements [5.3] lower the gravimetric and volumetric H2 storage capacities

significantly.

However, the intensive literature published after 1999 – a year of a new beginning in

the NaBH4 history –about sodium borohydride as an energy/hydrogen carrier, in particular

the work of Amendola et al. with the title “A safe, portable, hydrogen gas generator using

aqueous borohydride solution and Ru catalyst” [5.4], irrefutably showed that NaBH4 has

potential for portable applications, at least niche applications.

Sodium borohydride reacts with water to generate molecular hydrogen according to

the hydrolysis reaction shown in Eq.(5.1):


Ideal hydrolysis is attained for x =0 [5.5], but in practice excess of water is required to

account for the fact that the by-product (NaBO2.xH2O) can exist with various degrees of

hydration [5.6].

In this Chapter new and original data are reported on both generated and

simultaneously storage molecular hydrogen (in the liquid phase) in a batch reactor, under

high pressures, by the addition of small quantities of an organic polymer or surfactant to

the reactant solution before the hydrolysis reaction occur inside the reactor, the latter

within an appropriate amount of powder catalyst.

The key purpose of the polymer addition or the surfactant addition was to change the

polarity of the remaining solution, in order to increase the non-polar hydrogen-electrolyte

interactions; and hence, improving the affinity for storage H2 in the liquid phase by

solubility’s effects.

The interplay between the H2 solubility and the gravimetric H2 density is also an

objective point to report in this part of the work.


69


The lPOPS catalytic hydrolysis of sodium borohydride for hydrogen generation under

pressure comprises a series of experiments done in the two batch reactors MR and SR, with

internal free volumes of 369 cm3 and 229 cm3, respectively.

20 cm3 of reactant solution, prepared with 10 wt% NaBH4 and stabilized with 7 wt%

NaOH, was injected into one of the reactors (0.369 L and 0.229 L) using a syringe.

Another two solutions were as well prepared by addition of 0.25 wt% CMC and 0.25 wt%

SDS. Weighed amounts of catalyst, reused 160 times, were in the proportion of Ni-Ru

based catalyst/NaBH4: 0.4 g/g and had been previously stored in the conical bottom of the

two reactors tested. The adopted reactor bottom configuration – conical – enables non-

dispersible effects between the contacting powdered catalyst and the injected reactant

solution.

All the experimental tests were performed at 318 K, and after reaction completion,

magnetic stirring inside de reactor was allowed for 30 minutes to promote complete

saturation of H2 with the remaining by-product solution.


%100)n(H

)n(H)(H yield

ltheoretica2

exp22 ×= , (5.2)


theoretical amount of generated H2 assuming 100 % conversion of NaBH4 by applying the

ideal gas law to the final volume of gas inside the reactor. Special care was taken to correct

the varying free volume of gas inside the reactor due to the consumption of NaBH4 and

water.

The by-products of the hydrogen generation reactions were analyzed using X-Ray

Diffractometry (XRD). Suitable crystals were obtained by slow evaporation of water

solution at uncontrolled room temperature.


Plots in Figure 5.1 show a typical course of a hydrogen generation reaction, presented in

terms of the operating pressure as a function of time, for one single loading fuel solution,

respectively, in MR and SR batch reactors, both with conical bottom shapes.


70

0

5

10

15

20

0 1000 2000 3000 4000

Time / sec

Pre

ssu

re H

2 /

ba

r

Classic LPOPS-CMC Surfactant-SDS

0

5

10

15

20

0 20 40 60 80 100

(a)

0

5

10

15

20

25

30

0 1000 2000 3000 4000

Time / sec

Pre

ssu

re H

2 /

ba

r

Classic LPOPS-CMC Surfactant-SDS

0

5

10

15

20

25

30

0 20 40 60 80 100

(b)

Figure 5.1. Hydrogen generation by a single loading of 20 cm3 of reactant solution, at 45 ºC, with Ni-Ru based catalyst/NaBH4: 0.4 g/g: (a) MR batch reactor [0.369 L] and (b) SR batch reactor [0.229 L].

Table 5.1 reports the hydrogen yields for the three studied solutions, in the two batch

reactors. Completion was practically achieved in the classic hydrolysis and in the reactant

solution with the surfactant SDS. For the experiments with the polymer CMC, the

maximum hydrogen generated (evaluated by the maximum reached pressure inside the


71

batch reactor) was found to be lower than that predicted by the ideal gas law, and this

effect was more pronounced in the small reactor SR.

Table 5.1 – Hydrogen yield for the studied reactant solutions in the two batch reactors and prevision of respective H2 solubility effects, at 45 ºC.

without polymer CMC, 0.25 wt% SDS, 0.25 wt%

REACTOR 1 H2 yield, % 100 98 100

0.369 l Solubility effects, % 0 2 0

REACTOR 2 H2 yield, % 98 95 100

0.229 l Solubility effects, % 2 5 0

10 wt% NaBH4 and 7 wt% NaOH

A reasonable explanation for this behaviour can be related with solubility effects. In

fact, increasing pressure of the gas phase forces the H2 dissolution in the liquid phase that

remains inside the reactor [5.7].

Curiously, the by-product solution of the lPOPS catalytic hydrolysis of NaBH4 with

the CMC polymer, showed the highest capacities to store hydrogen gas in the liquid phase:

2 and 5%, respectively, in reactors MR and SR. Given that H2 is a non-polar gas, the result

found for the lPOPS-CMC hydrolysis was in some way expected. In fact, the addition of

the polymer CMC (0.25 wt %) probably changes the polarity of the final solution due to

the increase of non-polar C-H groups. Thus, we may state that the presence of CMC

enhances the affinity to store H2 in the liquid phase that remains inside the batch reactor,

under pressure, owing to the increase of the non polar hydrogen-electrolyte interactions.

Therefore, as it is well known, the effect of increasing the gas pressure enhances strongly

the gas dissolution in the liquid phase. Observing the values in Table 1 for the reactant

solution with CMC, we note an increase in solubility effects from 2 to 5% when the H2

pressure increased from 16.2 to 22.2 bars in the reactors MR and SR, respectively. Hence,

it can be concluded that increasing pressure of the gas phase forces the hydrogen

dissolution. As mention by Pinto et al. [5.7], this is an interesting result since gaseous

hydrogen was generated but also stored in the liquid phase.

The addition of polymer CMC and surfactant SDS results in a less Polar Organic

Polymeric Solution (lPOPS) and this was marked in the change of the overall conductivity

of the remaining solution inside the batch reactors. Table 5.2 shows the conductivity and

pH of the tested solutions before and after the reaction completion.


72

Table 5.2 – Values of conductivity and pH of the studied reactant solutions, before and after reaction completion.

20 mL reactant solution Conductivity pH Conductivity pH

10 wt% NaBH4, 7 wt% NaOH (mS/cm) (-) (mS/cm) (-)

Classic - no polymer addition 80,3 13,4 64,5 13,4

plus 0.25 wt% CMC 80,5 13,6 67,5 13,4

plus 0.25 wt% SDS 87,1 13,4 23,0 13,5

temperature

before the reaction after the reaction

298 K

As can be seen in the previous table, a decrease of approximately 15 mS/cm in the

value of conductivity was registered for both the reactant solutions: [10 wt% NaBH4 +

7wt% NaOH + 83wt% H2O] and [10wt% NaBH4 + 7wt% NaOH + 0.25wt% CMC +

82.75wt% H2O], before and after the reaction completion. A decrease of nearly more than

four times was observed for the reactant solution with 0.25wt% SDS (23 mS/cm). This

result was in some way expected due to the capability of SDS to form micelles. We believe

that this aggregate, usually with its hydrophilic head oriented to the solvent (water)

direction and hydrophobic tail leaning to its centre, had the capability to sequester H2 to the

micelle centre. Definitely that doesn’t seem to occur: the value of 0% of H2 due to

solubility effects (see Table 5.1) ‘speaks for itself’.


By-products suitable crystals of the hydrogen generation reactions of all types of

experiments reported on this chapter were analyzed by X-Ray Diffractometry. Very

interesting results were obtained for the chemical structures of sodium borohydride

hydrolysis by-product crystals:

i) For the classic reaction (5.1), the crystal structure of the by-product is a metaborate

dehydrate, NaBO2.2H2O, schematically presented in Figure 3.12 (see Chapter 3).

ii ) For the reagents presented in reaction (5.1) plus addition of 0.25wt% CMC, the

crystal structure of the by-product is a borate anhydrous, NaBO4, schematically presented

in Figure 5.2. The crystal structure reveals that the boron atoms are in a tetrahedral

configuration with four oxygen atoms, with B-O bond lengths within the interval 1.466 and

1.492 Å. The B-O interatomic distances are around the expected value for a tetrahedral

configuration 1.475 Å, which is appreciable higher than observed for triangular


73

configurations (1.365 Å) [5.8]. The sodium atoms have their valence distributed between

five Na-O bonds (bond lengths within 2.329 and 2400 Å). The mean length of the Na-O

bonds is 2.512 Å when the Na coordination number is seven and it is 2.450 Å when the

coordination number is six [5.9]. It is reasonable to assume that the bond length and the Na

coordination number decrease as the bond strength increases.

Figure 5.2. View along the a-axis of the crystal structure of sodium borate anhydrous, showing the BO4 tetrahedral arrangement and the Na bound network. Na in violet, O in red and B in pink.

iii ) Finally, for the reagents presented in reaction (5.1) plus addition of 0.25wt%

SDS, the crystal structure of the by-product reveals that the oxygen atoms adopt a

tetrahedral conformation around half of the boron atoms and a triangular conformation

around the other half. Tetrahedral B-O bond lengths range from 1.450 and 1.500 Å and

triangular B-O bond lengths from 1.355 and 1.387 Å, and are in agreement with the

interatomic distances observed in other structures [5.8]. The BO3 triangles and the BO4

tetrahedra share oxygen atoms so as to produce endless zigzag chains. The sodium atoms

form bonds with six oxygen atoms. The Na-O bond lengths are within 2.386 and 2448 Å,

which is slightly lower than it is expected for a Na coordination number of six [5.9].

Therefore, we speculate that a borate anhydrous, formed by NaBO3 and NaBO4 shared

molecules, schematically presented in Figure 5.3.

Figure 5.3. View along the a-axis of the crystal structure of sodium borate anhydrous, showing the BO3 triangular and the BO4 tetrahedral arrangements sharing oxygen atoms and the Na bound network. Na in violet, O in red and B in pink.


74

At this point, we highlight that adding just 0.25 wt% of CMC or 0.25wt% of SDS to

the reactants described in equation 1, leads to the formation of an anhydrous crystalline

borate. A pertinent question can now be asked: what happens to water molecules usually

associate to the classic catalytic hydrolysis of NaBH4 by-product? Unexpectedly, it seems

that both CMC polymer and SDS surfactant behave like desiccators of the NaBH4 by-

product hydrolysis! Particularly in the latter one, the absence for H2 solubility effects

observed in the remaining solution with 0.25 wt% SDS (see Table 5.1) can now be

explained by the capability of SDS to form micelles, enclosing water instead of hydrogen

gas, in a way similar to a clathrate.

Based on the above described achievements, it is thought that the results presented are

significant findings in terms of:

- NaBH4 by-product hydrolysis recyclability. In fact, the inexistence of H2O in the

crystalline borates mention in (ii ) and (iii ), marks effectively a cost reduction in the process

of recycling these by-products back to NaBH4, comparing the same stage of the process

but with the traditional NaBO2.2H2O. Actually, finding sustainable methods to recycle

hydrolysis NaBH4 by-product back to NaBH4, is a very important obstacle to overcome for

the adoption of NaBH4 as an hydrogen carrier of choice. Every step is important and our

finding is, perhaps, a significant contribution;

- Gravimetric H2 storage density. Beyond doubt, the ability to produce anhydrous

sodium borate (or metaborate) may be the key to increase the overall storage density of

systems based on sodium borohydride as the storage media [5.1,5.2,5.6]. The gravimetric

hydrogen storage density for the experiments reported in this work can be estimated

considering the number of water molecules (2+x) attached to the by-products found in the

crystals structures mentioned above. According to our experimental conditions, we may

consider the quantity of water in the ratios H2O/NaBH4: 4, 2 and 2 mol/mol, respectively,

for the crystals structures establish in (i), (ii ) and (iii ), as the minimum required amount (of

water) necessary for reaction (5.1) completion. In this way, the usable specific energy from

hydrogen, for the three sets of experiments reported in this work, can be evaluated on a

reactant-only basis. The plots in Figure 5.4 show the results for the gravimetric H2 storage

density, for the three reactant systems studied. It can be seen that the gravimetric H2

density is around 5.7 wt% for the classic reactant system and equal to 8.4 wt% and 8.7

wt%, respectively, for reactant systems with 0.25 wt% CMC and 0.25 wt% SDS.

We believe that it is possible to augment the solubility of hydrogen gas in the liquid

phase by-product (that remains inside the reactor under pressure) by small additions of


75

chemical materials, and this may be the key to increase the overall specific H2 energy of

systems based on NaBH4 as hydrogen storage media. As far as the authors are aware, this

issue has not been reported in the history of NaBH4 as energy/hydrogen carrier [5.10].

0123456789

reactant solution

Gra

vim

etr

ic H 2

de

nsity

/ %

"Classic" - no polymer additionLPOPS - 0.25 wt% CMCSurfactant - 0.25 wt% SDS

Figure 5.4. Gravimetric H2 storage density (reactants basis) for the three systems studied in this chapter.

The present results also indicate a remarkable improvement from a previous work by the

authors [5.7], in which, using the conventional excess of water method, performed also at

moderate pressures (up to 2.6 MPa), a gravimetric hydrogen density of 4.3 % was

achieved.

5.5 CONCLUSIONS

The capability of the liquid by-product (NaBO2.2H2O, metaborate dehydrate) to store

hydrogen gas by its dissolution was tested in this dissertation section. With that goal, small

amounts of an organic polymer, Carboxyl Methyl Cellulose (CMC), and a surfactant,

Sodium Dodecyl Sulphate (SDS), both in the form of fine powder, were added to the

reactant solution before its injection in the reactor. The results obtained show that the gas

dissolution in the liquid phase and the gravimetric hydrogen density increase with high H2

pressures, and are additionally enhanced by the polarity change (measured by

conductivimetry) of the remaining solution inside the reactor. As a consequence,

anhydrous borates were produced in the presence of these additives (CMC or SDS). We


76

believe that this finding may be the key to increase the overall storage density of systems

based on NaBH4 as hydrogen carrier.

To conclude, the results presented in this chapter showed that gravimetric H2 density

of the hydrolysis of sodium tetrahydroborate can be augmented to reach ≈ 9 wt%, by

adding small amounts of an organic polymer or surfactant to the stabilized reactant

solution. However, the eventual success of this new approach will depend upon the

development of a simple method of converting borates (BO2, BO3, BO4 groups) into

tetrahydroborate.

5.6 REFERENCES

[5.1] L. Schlapbach, A. Züttel, Hydrogen-storage materials for mobile applications, Nature 414 (2001) 353-358.

[5.2] U.B. Demirci, O. Akdim, P. Miele, Ten-year efforts and a no-go recommendation for sodium borohydride for on-board automotive hydrogen storage, Int. J. Hydrogen Energy

34 (2009) 2638-2645.


[5.4] S.C. Amendola, S.L. Sharp-Goldman, M.S. Janjua, N.C. Spencer, M.T. Kelly, P.J. Petillo, M. Binder, A safe, portable, hydrogen gas generator using aqueous borohydride solution and Ru catalyst, J. Power Sources 85 (2000) 186-189.


[5.6] E. Marrero-Alfonso, J.R. Gray, T.A. Davis, M.A. Matthews, Minimizing water utilization in hydrolysis of sodium borohydride: the role of sodium metaborate hydrates, Int. J. Hydrogen Energy 32 (2007) 4723-4730.


[5.8] W.H. Zachariasen, The crystal structure of monoclinic metaboric acid, Acta Cryst. 16 (1963) 385-389.

[5.9] M. Marezio, H.A. Plettinger, W.H. Zachariasen, The bond lengths in the sodium metaborate structure, Acta Cryst. 16 (1963) 594-595.

[5.10] U.B. Demirci, P. Miele, Sodium tetrahydroborate as energy/hydrogen carrier, its history, C. R. Chimie 12 (2009) 943-945.

77

CHAPTER 6

Nickel based bimetallic catalyst characterization Results after 200 reutilizations

This chapter focuses in the characterization of the nickel based bimetallic catalyst after 200 reutilizations. Textural properties based on nitrogen adsorption isotherms; surface morphology by scanning electron microscopy (SEM) coupled with EDS spectroscopy and X-ray photoelectron spectroscopy (XPS) analysis, were used to characterized the powder catalyst.

6.1 INTRODUCTION

Most published research papers in the area of this dissertation deal with catalytic material

used for hydrolyzing sodium borohydride. It is well known that catalysts suffer from

deactivation [6.1]. Nonetheless, very few papers studied the catalyst durability; among

these, a good example is the work reported by Kim et al. [6.2].

The deterioration of the catalyst activity is a very important issue, since the catalyst is

the key material for varying the amount of generated molecular hydrogen from NaBH4

hydrolysis reactions. The US DOE set technical targets for durability and operability of

catalysts, but the reasoning behind the targets set was not explained in their review [6.3].

It is the aim of this work to analyze the state of ‘health’ of the powder nickel based

bimetallic catalyst after it was used two hundred times. In fact, the catalyst was used in

several different types of kinetic experiments, as described in the previous chapters of this

dissertation, and always revealed good activity (by the low values in lag time and high

dtdP / slopes in the linear zone).

The fact that the catalyst was used more than 200 times motivated us to carry out a

detailed study of the catalyst activity. In truth, the catalyst showed good working efficiency

both under higher concentrations of hydride and inhibitor in the alkali NaBH4 hydrolysis

CHAPTER 6 Nickel based bimetallic catalyst characterization

78

with excess of water (even in the experiments with refuelling fresh NaBH4 solution), and

with alkali free hydrolysis with 2 < H2O/NaBH4, mol/mol < 4. It was also verified that the

influence of reactor design (bottom shape) affects the catalyst activity; in particular, the

reaction vessel with conical bottom shape promoted an intensification of the catalyst

activity, by reducing the lag time of the reaction, in comparison with the flat bottom shape.

6.2 CATALYST CHARACTERIZATION

The Ni-Ru based powdered catalyst after 200 times of reuse was characterized by textural

properties based on nitrogen adsorption isotherms; by surface morphology using scanning

electron microscopy (SEM) coupled with EDS spectroscopy and by X-ray photoelectron

spectroscopy (XPS). A brief description of these instrumental analysis techniques and the

results obtained are given in the following sections.

6.2.1 Textural properties

Our concern here is with the internal surface area of the porous Ni-Ru based solid catalyst,

and with the nature of the pores giving rise to this area. In most porous solids the external

surface is a negligible fraction of the total surface, but if relevant it may readily be

estimated by measuring the permeability of gas through a bed of the solid, or by

sedimentation. The parameters of interest are then the internal surface area and the pore

volume, from which an average pore radius may be obtained, and the pore size

distribution. Knowledge of the pore size distribution is obtainable by mercury porosimetry.

In this technique, mercury is forced under pressure into the pores, and the greater the

pressure, the smaller the pores to which the mercury obtains access. By sensitively

measuring the change in volume of the solid plus the mercury with increasing pressure, a

picture of the pore size distribution is obtained.

Useful information is also gained by measuring the physical adsorption of gases on

porous solids. It is known that the monolayer capacity of a non-porous solid, measured by

chemisorption, or by physical adsorption well above the boiling point of the adsorbing gas

(usually N2), can be easily translated into a surface area. However with porous solids, and

using temperatures close to the boiling point of the gas, so that multilayer adsorption

occurs, several forms of isotherms besides the Langmuir type can be observed. The Type

IV is of great interest. This isotherm usually shows a hysteresis loop, that is, the isotherm

does not follow the same path in the desorption as it does in adsorption. The reason for this


79

is that evaporation of condensed gas in fine pores does not occur as easily as its

condensation; this is because a molecule evaporating from a highly curved meniscus has a

higher probability of recondensing than one evaporating from a plane surface. This effect

was discussed many years ago by Lord Kelvin who devised the following equation to

describe the effect:

φγcos

2)/ln( 0 rRT

VPP −= , (6.1)

where V is molar volume of the liquid, γ its surface tension, r the pore radius and φ the

contact angle (usually taken to be zero); the other symbols have their usual significance.

The relative pressure at which condensation will occur in a pore of a given size can thus be

determined, and the isotherm then used to obtain a pore size distribution. Hysteresis loops

vary greatly in shape, and these too have also been classified in literature [6.4].

The textural characterization of the Ru-Ni based powdered catalyst (after 200

reutilizations), including the determination of surface areas (SBET), was based on the N2

adsorption isotherms, using a Quantachrome Instruments Nova 4200e apparatus. Prior to

the analysis, the sample (0.127 g) was degassed at 160 ºC for 3 h. Nitrogen was used as

adsorbate at liquid nitrogen temperature (77 K). Pore size distribution was obtained from

the desorption branch of the isotherms using the Barrett, Joyner and Halenda (BJH)

method. The micropore volumes and mesopore surface areas were determined by the t-

method.

The adsorption and desorption isotherms of the powder Ni-Ru based catalyst 200 times

reused are displayed in Figure 6.1. The result shows that the N2 isotherm shape are not of

type IV, at least for P/P0<0.986. Hence the validity of BJH method is discussable for this

powder material. The plot on Figure 6.2 illustrates the pore size distribution. As shown, the

pore radius peak shifts to a smaller size region, in the range 17-23 Å.


80

0

20

40

60

80

100

120

0 0,2 0,4 0,6 0,8 1

Relative pressure / P/P0

Vo

lum

e ad

sorb

ed /

cm3 ST

P g-1

Figure 6.1. Nitrogen isotherms of powder Ni-Ru based catalyst 200 times reused.

0,0E+00

4,0E-03

8,0E-03

1,2E-02

1,6E-02

2,0E-02

0 50 100 150 200 250

pore radius / Å

dV

(r)

/ cm

3 g-1

Å-1

Figure 6.2. Pore size distributions of powder Ni-Ru based catalyst 200 times reused.

Table 6.1 summarizes the textural properties of powder Ni-Ru based catalyst 200 times

reused.

Table 6.1 - Textural properties of powder Ni-Ru based catalyst 200 times reused sample.

SBET / m2g-1 Vµ / cm3 g-1 Sext

/ m2 g-1 Vp / cm3 g-1 58 0 58 0.15

SBET = BET surface area; Vµ = micropore volume by t-method; Sext = (mesopore + macropore) surface areas by t-method; Vp = total pore volume, for P/P0=0.986.


81

It should be noted that the rate of a catalytic reaction should be proportional to the

surface area of the catalyst, provided that the transfer of reactants to or of products from

the surface is not the slow step. It is therefore normally desirable to get as much surface

area as possible into a given volume.

Analysing the data plotted in Fig. 6.2, we may conclude that the pores through the

particles catalyst sample, assuming microspheres of uniform size, lies between 4 and 20

nm in diameter. This means that the pore size in our catalyst, after 200 reutilizations,

belongs to the group of mesopores. Would pores below 2 nm in width, i.e., micropores be

expectable?

If micropores existed, can they be surface closed by effects of particle agglomeration?

The values of surface areas, by BET and t methods, reveal 58 m2/g; surprisingly, this is

not a large surface area for a fine powder catalyst [6.4]. In fact Vµ = 0 cm3/g by t-method.

6.2.2 Morphology

Surface morphological observations were obtained at CEMUP (Centro de Materiais da

Universidade do Porto). The Scanning Electron Microscopy (SEM imaging) coupled with

EDS (analysis) spectroscopy was performed using a unit FEI Quanta 400 FEG ESEM /

EDAX Genesis X4M operating at 15 kV in low vacuum mode (LVSEM for uncoated non

conductive sample). The powder sample was prepared by simple dispersion over a double

side adhesive carbon tape.

Figure 6.3 shows SEM micrographs of the powder Ni-Ru based catalyst 200 times

reused, at magnifications up to 100 000X. SEM images at higher magnification showed

distortions due to slight magnetic properties of the catalyst sample.

In the micrographs of Fig.6.3, from (a) to (b), the catalyst sample in the bulk form

seems that can be broken-up into smaller particles until reaching a consistency similar to

that of a fine dust. Image 6.3(b) show a Ni-Ru based particle that is about 100 µm in

vertical length and a background of smaller ones of about 1-25 µm in diameter. Going

from Fig.6.3 (b) to (e), the magnetic properties of Ni-Ru based appear to cause some

agglomeration of the smaller particles. Therefore, closer examination of the particles

surface in Fig.6.3 (c) shows a finer reticulated structure, interconnecting more or less

spherical particles.


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(a)

(b)

(c)

(d)

(e)

Figure 6.3. SEM micrographs of powder Ni-Ru based catalyst, 200 times reused. Magnifications between 200 and 100 000X, increasing from (a) to (e).


83

Going from Fig.6.3 (d) to (e), it may be assumed that mesopores (and macropores)

exist between these ‘microspheres’, which form a three-dimensional structure.

EDS spectroscopy was used to analyse the chemical content of Ni-Ru based catalyst

sample. Figures 6.4 – 6.6 shows the SEM image coupled with EDS spectrum of Ni-Ru

based catalyst 200 times reused, at specific magnifications of 200X, 1 500X and 10 000X,

respectively.

Figure 6.4. SEM image coupled with EDS spectrum of Ni-Ru based catalyst 200 times reused and a magnification of 200X.

The EDS spectra show that nickel (Ni) is the main constituent of the powder Ni-Ru

based catalyst 200 times reused. Small portions of silicium (Si) and sodium (Na) were

detectable in the catalyst sample at magnification of 1500X (see Fig.6.5).

In Figure 6.6, the amount (weight percentage) of ruthenium (Ru) element depends

directly on the magnitude of the surface area selected to EDS analysis. In fact, for

particular E2, E3 and E4 areas, the corresponding EDS spectra have shown different

quantities of Ru.


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Figure 6.5. SEM image coupled with EDS spectrum of Ni-Ru based catalyst 200 times reused and a magnification of 1500X.

Therefore, the amount of Ru recorded on EDS spectrum is somewhat related with the

more brilliant or dense area visualized on SEM micrographs, and this calls for a non

homogeneous distribution of Ru element on the sample. Is this an added characteristic of

Ru-Ni based catalyst after 200 times reused?

At this time, we may assume that the continuous reuse of Ni-Ru based catalyst

enhance some loss of Ru from the initial metal matrix (Ru-Ni based, 0 times used). Hence,

due to the magnetic properties of Ru-Ni based, the natural tendency of these lost Ru

elements is to agglomerate and then form denser areas on the catalyst sample, which

becomes richer in Ru element.

6.2.3 XPS analysis

In order to obtain additional information about the surface structure/composition of our

200 times reused catalyst, X-ray Photoelectron Spectra (XPS) analyses were performed at

CEMUP (Centro de Materiais da Universidade do Porto) using a VG Scientific ESCALAB

200A spectrometer with PISCES software for data acquisition and analysis.


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Figure 6.6. SEM image coupled with EDS spectrum, at three different selected areas, of Ni-Ru based catalyst 200 times reused and a magnification of 10 000X.

For the analysis, an achromatic Al (Kalfa) X-ray source operating at 15kV (300 W)

was used, and the spectrometer, calibrated with reference to C1s (285 eV), was operated in

CAE mode with 20 eV pass energy. Data acquisition was performed with a pressure lower

than 1E-6 Pa. Spectra analysis was performed using peak fitting with Gaussian-Lorentzian

peak shape and Shirley type background subtraction (or linear taking in account the data).

The powder sample, with a shape of 10 mm diameter pellet (see Fig.6.7), was

appropriately fixed on a copper plate before inserting it in the spectrometer for XPS

analysis.


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Figure 6.7. Photograph of Ni-Ru based catalyst 200 times reused pellet, with 10 mm in diameter, for use in XPS analysis.

Table 6.2 details the conditions in which XPS analysis was carried out.

Table 6.2 - Specific conditions for XPS analysis of Ni-Ru based catalyst 200 times reused.

Regions Pass Energy Step / eV Dwell Time / ms Survey 50 1 200 C 1s – Ru 3d 20 0.1 1000 O 1s 20 0.1 1000 Ni 2p 20 0.1 1000

Tilt=0 | Source: Al (Kalfa) X-ray | Eo = 15kV (300W) | Charge Corr = NA | Ref: C 1s=285 eV

Figure 6.8, shows a survey XPS spectrum of the studied sample. A small amount of Na

was detected.

Figure 6.8. A survey XPS spectrum of Ni-Ru based catalyst 200 times reused.

Deconvolution XPS spectra are sequentially presented for C1s – Ru 3d, O1s and Ni 2p

(2p1,2p3) in the plots of Figure 6.9. A common behaviour observed on these spectra is the

presence of two dominant peaks. For the spectrum analysis, peak fitting with Gaussian-


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Lorentzian peak shape and Shirley type background subtraction (or linear taking in account

the data) were used. In the particular case of C1s - Ru 3d5 spectrum, the existence of two

groups of Ru 3d5 peaks displaced by effects of differential electric charge was assumed.

C 1s – Ru 3d

O 1s

Ni 2p (2p1,2p3)

Figure 6.9. XPS spectra for Ni-Ru based powdered catalyst 200 times reused.

Table 6.3 summarises the XPS analysis results of the Ni-Ru based catalyst 200 times reused.


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Table 6.3 - Results of XPS analysis of Ni-Ru based catalyst 200 times reused.

Element Peak range Area Sens. Factor At / % C 1s synthesis 13285 1 13.57 O1s synthesis 187038 2.93 65.20

Ni 2p3 synthesis 283316 14.61 19.81 Ru 3d5 synthesis 10269.3 7.39 1.42

At = total surface area.

The XPS analysis showed that nickel (Ni 2p3) is the main metal surface constituent of

our bimetallic powdered catalyst 200 times reused, 13.57 % At. Ruthenium (Ru 3d5)

contribution is about 1.42 % At. The XPS data confirmed the increased in Ru

concentration (Ru3d5) from 0.74 to 1.42 At %. A slight increase in area (At %) is

suggested which is thought to be due to segregation of ruthenium as a result of the

extensive use in the catalyzed hydrolysis.

6.3 CATALYST ACTIVITY ANALYSIS AFTER 200 REUTILIZATIONS

This section intends to characterize the catalyst after 200 reutilizations. The state of

‘health’ of our Ni-Ru based powdered catalyst (200 times reused) is checked by evaluating

its activity, in terms of yield, lag time and dtdP / slopes in the linear zone, at two different

stages of reutilization. Two experiments, of the topic of alkali hydrolysis of NaBH4, were

chosen to illustrate our analysis.

Plots in Figure 6.10 shows the hydrogen generation of 10 mL of a reactant solution

with concentration of 10 wt% NaBH4 and 7 wt% NaOH by weight, performed in reactor

LR (646 cm3) with a proportion of Ni-Ru based catalyst/NaBH4: 0.4 g/g (≈ 0.43 g), when

the catalyst was 28 times and 200 times reused. No stirring was performed in the solutions

inside the reactor. Both reactions were performed at room temperature, without

temperature control: the reaction with catalyst reused 28 times was carried out in the

summer of 2008 (at 27 ºC) and the reaction with catalyst reused 200 times in the middle of

the spring of 2009 (25 ºC).

Table 6.4 shows the values of lag time and dP/dt obtained from the curves presented in

Figure 6.10.

The results in Table 6.4 (and in Figure 6.10) revealed that our nickel-based bimetallic

catalyst reused 200 times is ‘healthy’, although some deterioration in terms of H2 yield and

rate is evident (note that the two reactions plotted in Figure 6.10 employ a catalyst

separated by 172 uses).


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Table 6.4 - Results for yield, lag time and dtdP / slope in hydrogen generation of 10 mL of 10wt% NaBH4 and 7wt% NaOH solution, at two different stages of Ni-Ru based catalyst reutilization (for 28 and 200 times reused), in batch reactor LR (646 cm3).

Ni-Ru based catalyst yield / % lag time / s dtdP / slope* / bar/s 28 times reused 100 20 2.48E-02 200 times reused 91 21 7.59E-02

* calculated in the linear zone.

0

1

2

3

4

5

0 200 400 600 800

time / s

Pre

ssur

e /

ba

r

28 times reused catalyst

200 times reused catalyst

Theoretical yield

Figure 6.10. Hydrogen generation plots with a NaBH4 concentration of 10%, an inhibitor concentration of 7%, for two different stages of catalyst reutilization (28 times reused and 200 times reused). The reactions, for 10 cm3 of reactant solution, were performed in LR batch reactor (646 cm3) with a proportion of Ni-Ru based catalyst/NaBH4: 0.4 g/g, at room temperature of ≈ 25 ºC.

In terms of yield, the results found in Table 6.4 shows deterioration by a factor of 1.1,

XX 20028 1.1 ηη = , this means that the average loss in yield per each utilization of the catalyst

is approximately 0.6 %. In terms of hydrogen generation rate, a larger factor was found in

the universe of 172 reutilizations: 3.3, i.e., XX dtdPdtdP 20028 )/(3.3)/( = in the linear zone;

hence a loss of 1.9% per reutilization can be estimated for the rate value. With respect to

lag time, identical values were found in the two experiments, indicating that the catalyst

can be reused several times without affecting lag time.

6.4 CONCLUSIONS

The study of catalyst durability on catalyzed hydrolysis of sodium borohydride is essential

from an application point of view. Few works on this topic are available in the literature. In

the present chapter, an effort was made to characterize the powder nickel-based bimetallic


90

catalyst, used in two different schemes of NaBH4 hydrolyzing (alkali hydrolysis and alkali

free hydrolysis) and after 200 reutilizations.

The textural characterization of the Ru-Ni based powdered catalyst (200 reused)

showed an N2 isotherm shape different from type IV, at least for P/P0<0.986, evaluated by

BJH method. The pore size distribution, obtained from the desorption branch of the

isotherm, gave pore radius peak shifts in the range 17-23 Å, showing an abundant presence

of mesopores in the catalyst. This corroborate with the values found for surface areas by

BET and t methods of 58 m2/g. The t-method also revealed the absence of micropores

volume in the 200 times reused powder catalyst.

Surface morphology analysis by using scanning electron microscopy (SEM) coupled

with EDS spectroscopy revealed that nickel is the main constituent of the powder Ni-Ru

based catalyst 200 times reused. The XPS study showed that nickel (Ni 2p3) is the main

metal surface element (13.57 % At.) and ruthenium (Ru 3d5) contributed with 1.4 % At.

The catalyst activity was checked by confronting the values of yield, lag time and

dtdP / slopes (in the linear zone, of a graphical representation of reactor inside pressure as

function of time). In two similar experiments for hydrogen generation by alkali hydrolysis

of NaBH4, but performed at two different stages of catalyst reutilization (which differ from

each other in 172 times), the results revealed some deterioration of the catalyst activity (the

dtdP / slopes are lower, as expected, after such a long time utilization).

Finally, we may conclude that the powder Ni-Ru based catalyst, after 200 applications

for hydrolyzing NaBH4, looses approximately 0.6 % in yield and 1.9% in rate, per

reutilization, but still preserves the lag time required to observe an increase in H2 pressure.

For this reason, since no great major deterioration was found in the catalyst activity after it

was used 200 times - in truth, it still works at very reasonable H2 generation rates - we can

conclude that its durability is rather long. A possible explanation for this active catalyst,

comparing with similar work done by Pinto el al. [6.5], is the presence of 1.4 wt% of

ruthenium, a noble Group VIII metal.

6.5 REFERENCES

[6.1] J.H. Wee, A comparison of sodium borohydride as a fuel for proton exchange membrane fuel cells and for direct borohydride fuel cells , J. Power Sources 155 (2006) 329-339.

[6.2] J.H. Kim, K.T. Kim, Y.M. Kang, H.S. Kim, M.S. Song, J.Y. Lee, Study on degradation of filamentary Ni catalyst on hydrolysis of sodium borohydride , J. Alloys Comp. 379 (2004) 222-227.


91


[6.4] G.C. Bond, “Heterogeneous catalysis: principles and applications”, Oxford Chemistry Series, 1974.



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93

CHAPTER 7

Conclusion This chapter is devoted to conclusions and suggestions for future work.

Conclusions drawn from the results indicate that the reused ruthenium nickel based catalyst is capable of catalyzing the sodium borohydride solutions at a sufficient rates, in the two schemes presented in this dissertation for comparison – alkali hydrolysis and alkali free hydrolysis of sodium borohydride for hydrogen generation under pressure.

7.1 CONCLUSIONS

This section provides a summary of the primary research results. The sequence of

appearance is the one of the body of the manuscript.

Experimental techniques and apparatus

Description of the materials used in all the experimental work, including the nickel based

bimetallic catalyst, is given.

The plan for studied the catalytic hydrolysis of sodium borohydride under pressure, in

three batch reactors, with different bottom geometries, is available.

To study the influence of reactor bottom shape on the hydrogen generation rates - this

constitutes one of the originalities of the present dissertation, two different bottom

geometries were designed – one flat and the other conical. The latter has the purpose of

enabling non-dispersible effects during the contact of the catalyst with the reactant

solution.

Alkali hydrolysis, for sodium borohydride kinetic experiments

An extensive experimental work was carried out with the objective of studying various

effects in hydrogen generation rate, yield and lag time, by catalytic alkali hydrolysis of

sodium borohydride. It was concluded that:

CHAPTER 7 Conclusion

94

- increasing the temperature of the reaction medium, increases the rate of H2 generation;

- the reaction rate increases with increasing the hydride concentration up to 20 wt% of

NaBH4. Above this value, a slight decrease on both H2 and yield was found;

- the reaction rate is greatly enhanced by the increase of the NaOH concentration, ensuring

good efficiency of hydrogen generation;

- the reactor bottom shape affects the course of hydrogen generation. It was found that the

conical bottom geometry leads to an increase on reaction rate, in the absence of H2 lag

time;

- the Ni-Ru based catalyst has a good performance in refueling experiments. In fact, it was

found that the catalyst tends to perform better, with a higher H2 rate, after the first loading

of reactant (the amount of catalyst remains the same from the first to the last loading);

- a vigorous stirring of the solution inside the batch reactor, during the experiments with

successive fuel loadings, reveals minor reaction induction time and a slight higher yield in

hydrogen generation. This indicates possible mass transfer limitations during the course of

reaction between the fuel and the active sites of the catalyst.

The x-ray diffractions analyses reveal a sodium metaborate dehydrated, NaBO2.2H2O,

as the main by-product.

Alkali free hydrolysis, for sodium borohydride kinetic experiments

Chemical hydrides have the potential to meet certain H2 storage targets only if the water

consumption is minimized. The main purpose of the alkali free hydrolysis of NaBH4

experiments reported in this dissertation was to produce pure hydrogen gas with very high

gravimetric and volumetric densities.

The alkali free hydrolysis under pressure, when the operating pressure is > 1 MPa, for

a system with H2O/NaBH4: 4 mol/mol and Ni-Ru based/NaBH4: 0.4 g/g, performed in a

batch reactor with conical bottom shape (229 cm3), produced H2 almost at 100%

conversion of NaBH4, and hydrogen was delivered at a fast rate without significant lag

time. In terms of gravimetric hydrogen density a value of 6.3% was found (this value does

not take into account the hardware of the system).

lPOPS hydrolysis, for sodium borohydride kinetic experiments

The capability of the liquid by-product (NaBO2.2H2O, metaborate dehydrate) to store

hydrogen gas by its dissolution was tested in this dissertation section. With that goal, small

amounts of an organic polymer, Carboxyl Methyl Cellulose (CMC), and a surfactant,


95

Sodium Dodecyl Sulphate (SDS), both in the form of fine powder, were added to the

reactant solution before its injection in the reactor. The results obtained show that the gas

dissolution in the liquid phase and the gravimetric hydrogen density increase with high H2

pressures, and are additionally enhanced by the polarity change (measured by

conductivimetry) of the remaining solution inside the reactor. As a consequence,

anhydrous borates were produced in the presence of these additives (CMC or SDS). We

believe that this finding may be the key to increase the overall storage density of systems

based on NaBH4 as hydrogen carrier. However, the eventual success of this new approach

will depend upon the development of a simple method of converting borates (BO2, BO3,

BO4 groups) into tetrahydroborate.

Nickel based bimetallic catalyst characterization after 200 times reuse

Catalyst durability on catalyzed hydrolysis of sodium borohydride is essential from an

application point of view.

The textural characterization of the Ru-Ni based powdered catalyst (200 reused)

showed an N2 isotherm shape different from type IV, at least for P/P0<0.986, evaluated by

BJH method. The pore size distribution, obtained from the desorption branch of the

isotherm, gave pore radius peak shifts in the range 17-23 Å, showing an abundant presence

of mesopores in the catalyst. This corroborates with the values found for surface areas by

BET and t methods of 58 m2/g. The t-method also revealed the absence of micropores

volume in the 200 times reused powder catalyst.

Surface morphology analysis by using scanning electron microscopy (SEM) coupled

with EDS spectroscopy revealed that nickel is the main constituent of the powder Ru-Ni

based catalyst 200 times reused. The XPS study showed that nickel (Ni 2p3) is the main

metal surface element (13.57 % At.) and ruthenium (Ru 3d5) contributed with 1.4 % At.

The catalyst activity was checked by confronting the values of yield, lag time and

dtdP/ slopes (in the linear zone, of a graphical representation of reactor inside pressure as

function of time). In two similar experiments for hydrogen generation by alkali hydrolysis

of NaBH4, but performed at two different stages of catalyst reutilization (which differ from

each other in 172 times), the results revealed some deterioration of the catalyst activity (the

dtdP/ slopes are lower, as expected, after such a long time utilization).

Finally, we may conclude that the powder Ni-Ru based catalyst, after 200 applications

for hydrolyzing NaBH4, looses approximately 0.6 % in yield and 1.9% in rate, per

reutilization, but still preserves the lag time required to observe an increase in H2 pressure.


96

For this reason, since no great major deterioration was found in the catalyst activity after it

was used 200 times - in truth, it still works at very reasonable H2 generation rates - we can

conclude that its durability is rather long. A possible explanation for this activity of the

catalyst is the presence of 1.4 wt% of ruthenium, a noble Group VIII metal.

7.2 SUGGESTIONS FOR FUTURE WORK

The following is a list of the objectives that would, perhaps, be most significant as further

development of the present research:

I – Development of a reaction model that reflects the reaction rate as a function of

concentration, pressure, and temperature, model which is currently not available in the

literature. In fact, a model capable of predicting the hydrolysis rate for hydrolyzing

systems under increasing pressure, would be worthwhile principally for systems similar to

that developed in this dissertation;

II - Rate data is needed for NaBH4 concentrations greater than 30 wt%. Systems intended

to exceed the DOE 2010/2015 objective will require information about how solutions

behave at high NaBH4 concentrations. Additional information is also needed at low

temperatures, less than 20 ºC, to address start-up issues associated with hydrogen

generators or fuel cells operating in cold environments;

III – Data on solubility of H2 in aqueous borate systems is needed for storage molecular

hydrogen in liquid phase. Working at high H2 pressures, where the hydrogen generated is

allowed to build up within the closed system, facilitate dissolution of hydrogen by

solubility effects. Hence, it is possible envisage a system that generates and simultaneously

stores molecular hydrogen in the reminiscent liquid phase inside the reactor. Indeed

information about the amount of hydrogen dissolved in the solution with several

concentrations of hydride and of inhibitor, which can be estimated trough Henry’s Law for

the range of pressure and temperatures of operation, is necessary to carry out.

IV – Knowing materials (chemicals) which enhance the solubility of hydrogen gas in

liquid phase, under moderate pressures, is of great value for hydrogen storage in liquid

phase.

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